Semiconductor device and method for manufacturing the same

ABSTRACT

A semiconductor device with reduced parasitic capacitance is provided. The semiconductor device includes a first insulating layer; a first oxide layer over the first insulating layer; a semiconductor layer over the first oxide layer; a source electrode layer and a drain electrode layer over the semiconductor layer; a second insulating layer over the first insulating layer; a third insulating layer over the second insulating layer, the source electrode layer, and the drain electrode layer; a second oxide layer over the semiconductor layer; a gate insulating layer over the second oxide layer; a gate electrode layer over the gate insulating layer; and a fourth insulating layer over the third insulating layer, the second oxide layer, the gate insulating layer, and the gate electrode layer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an object, a method, or a manufacturingmethod. Further, the present invention relates to a process, a machine,manufacture, or a composition of matter. In particular, the presentinvention relates to, for example, a semiconductor device, a displaydevice, a light-emitting device, a power storage device, an imagingdevice, a driving method thereof, or a manufacturing method thereof. Inparticular, one embodiment of the present invention relates to asemiconductor device or a method for manufacturing the semiconductordevice.

In this specification and the like, a semiconductor device generallymeans a device that can function by utilizing semiconductorcharacteristics. A transistor and a semiconductor circuit areembodiments of semiconductor devices. In some cases, a storage device, adisplay device, or an electronic device includes a semiconductor device.

2. Description of the Related Art

A technique by which transistors are formed using semiconductor filmsformed over a substrate having an insulating surface has been attractingattention. The transistor is used in a wide range of electronic devicessuch as an integrated circuit (IC) or an image display device (displaydevice). As semiconductor thin films that can be used for thetransistors, silicon-based semiconductor materials have been widelyknown, but oxide semiconductors have been attracting attention asalternative materials.

For example, a transistor whose active layer includes an amorphous oxidesemiconductor containing indium (In), gallium (Ga), and zinc (Zn) isdisclosed in Patent Document 1.

REFERENCE Patent Document

[Patent Document 1] Japanese Translation of PCT InternationalApplication No. H11-505377

SUMMARY OF THE INVENTION

In miniaturization of a semiconductor element, the parasitic capacitancegenerated in the vicinity of a transistor is a major problem.

In the case where parasitic capacitance exists in a channel (e.g.,between a source electrode and a drain electrode) and the vicinity ofthe channel, a time for charging the parasitic capacitance is needed inthe transistor operation; thus, not only the responsiveness of thetransistor but the responsiveness of the semiconductor device islowered.

It becomes more difficult to control various steps of manufacturingtransistors (in particular, film formation, processing, and the like) asthe miniaturization advances, and variations due to the manufacturingprocess significantly affect transistor characteristics and reliability.

In miniaturization, it becomes more difficult to process the transistorsowing to higher accuracy required for the processing.

Thus, an object of one embodiment of the present invention is to reducethe parasitic capacitance in the vicinity of a transistor. Anotherobject is to provide a semiconductor device with favorable electricalcharacteristics. Another object is to provide a semiconductor devicewith high reliability. Another object is to reduce variations incharacteristics caused by a manufacturing process of a transistor or asemiconductor device. Another object is to stabilize a manufacturingprocess of a transistor. Another object is to provide a semiconductordevice including an oxide semiconductor layer having few oxygenvacancies. Another object is to provide a semiconductor device that canbe manufactured in a simple process. Another object is to provide asemiconductor device with a structure in which the density of interfacestates in the vicinity of the oxide semiconductor layer can be reduced.Another object is to provide a semiconductor device with low powerconsumption. Another object is to provide a novel semiconductor deviceor the like. Another object is to provide a method for manufacturing thesemiconductor device.

Note that the descriptions of these objects do not disturb the existenceof other objects. In one embodiment of the present invention, there isno need to achieve all the objects. Other objects will be apparent fromand can be derived from the description of the specification, thedrawings, the claims, and the like.

One embodiment of the present invention is a semiconductor deviceincluding a first insulating layer; a first oxide layer over the firstinsulating layer; a semiconductor layer over the first oxide layer; asource electrode layer and a drain electrode layer over thesemiconductor layer; a second insulating layer over the first insulatinglayer; a third insulating layer over the second insulating layer, thesource electrode layer, and the drain electrode layer; a second oxidelayer over the semiconductor layer; a gate insulating layer over thesecond oxide layer; a gate electrode layer over the gate insulatinglayer; and a fourth insulating layer over the third insulating layer,the second oxide layer, the gate insulating layer, and the gateelectrode layer. The second insulating layer includes a region incontact with the side surfaces of the first oxide layer, thesemiconductor layer, the source electrode layer, and the drain electrodelayer. The top surface of the second insulating layer is at the samelevel as the top surfaces of the source electrode layer and the drainelectrode layer. The second oxide layer includes a region in contactwith the side surfaces of the first oxide layer, the source electrodelayer, the drain electrode layer, the second insulating layer, and thethird insulating layer.

Another embodiment of the present invention is a method formanufacturing a semiconductor device including the steps of forming afirst insulating layer; forming a first oxide film over the firstinsulating layer; forming a semiconductor film over the first oxidefilm; forming a first conductive film over the semiconductor film;forming a first mask over the first conductive film; partly etching thefirst conductive film with the first mask to form a first conductivelayer in an island shape; partly etching the first oxide film and thesemiconductor film with the first mask and the first conductive layerserving as a mask to form a first oxide layer and a semiconductor layerin island shapes; forming a second insulating film over the firstinsulating layer and the first conductive layer; performing a chemicalmechanical polishing process on the second insulating film until thefirst conductive layer is exposed to form a second insulating layer;forming a third insulating film over the first conductive layer and thesecond insulating layer; forming a second mask over the third insulatingfilm; partly etching the third insulating film with the second mask toform a source electrode layer, a drain electrode layer, and a thirdinsulating layer; forming a second oxide film over the third insulatinglayer and the semiconductor layer; forming a fourth insulating film overthe second oxide film; forming a second conductive film over the fourthinsulating film; and performing a chemical mechanical polishing processon the second conductive film, the fourth insulating film, and thesecond oxide film to form a second oxide layer, a gate insulating layer,and a gate electrode layer.

It is preferable that first heat treatment be performed after thesemiconductor film is formed, that a fourth insulating layer containingoxygen be formed over the third insulating layer, the second oxidelayer, the gate insulating layer, and the gate electrode layer, that amixed layer of the third insulating layer and the fourth insulatinglayer be formed when the fourth insulating layer is formed and, at thesame time, oxygen be added to the mixed layer or the first insulatinglayer, and that second heat treatment be performed to diffuse the oxygento the semiconductor layer.

It is preferable that the third insulating film be an insulating filmcontaining oxygen, and that the fourth insulating layer be formed by asputtering method with an oxygen gas.

It is preferable that the third insulating film be a silicon oxide film,and that the fourth insulating layer be formed by a sputtering methodwith an oxygen gas of 50 vol % or higher and an aluminum oxide target.

It is preferable that the second heat treatment be performed at atemperature higher than or equal to 300° C. and lower than or equal to450° C.

Another embodiment of the present invention is a method formanufacturing a semiconductor device including the steps of forming afirst insulating layer; forming a first oxide film over the firstinsulating layer; forming a semiconductor film over the first oxidefilm; forming a first conductive film over the semiconductor film;forming a second insulating film over the first conductive film; forminga first mask over the second insulating film; partly etching the secondinsulating film and the first conductive film with the first mask toform a first conductive layer and a second insulating layer; forming asecond mask over the second insulating layer and the semiconductor film;partly etching the second insulating layer, the first conductive layer,the first oxide film, and the semiconductor film with the second mask toform a first oxide layer, a semiconductor layer, a source electrodelayer, a drain electrode layer, and a third insulating layer; forming asecond oxide film over the first insulating layer, the third insulatinglayer, and the semiconductor layer; forming a third insulating film overthe second oxide film; forming a second conductive film over the thirdinsulating film; forming a third mask over the second conductive film;and partly etching the second oxide film, the third insulating film, andthe second conductive film with the third mask to form a second oxidelayer, a gate insulating layer, and a gate electrode layer.

It is preferable that first heat treatment be performed after thesemiconductor film is formed, that a fourth insulating layer containingoxygen be formed over the first insulating layer, the third insulatinglayer, and the gate electrode layer, that a first mixed layer of thefirst insulating layer and a fourth insulating layer and a second mixedlayer of the third insulating layer and the fourth insulating layer beformed when the fourth insulating layer is formed and, at the same time,oxygen be added to the first mixed layer, the second mixed layer, thefirst insulating layer, or the second insulating layer, and that secondheat treatment be performed to diffuse the oxygen to the semiconductorlayer.

It is preferable that each of the first insulating film and the secondinsulating film be an insulating film containing oxygen, and that thefourth insulating layer be formed by a sputtering method with an oxygengas.

It is preferable that each of the first insulating film and the secondinsulating film be a silicon oxide film, and that the fourth insulatinglayer be formed by a sputtering method with an oxygen gas of 50 vol % orhigher and an aluminum oxide target.

It is preferable that the second heat treatment be performed at atemperature higher than or equal to 300° C. and lower than or equal to450° C.

Another embodiment of the present invention is a method formanufacturing a semiconductor device including the steps of forming afirst insulating layer; forming a first oxide film over the firstinsulating layer; forming a semiconductor film over the first oxidefilm; forming a first conductive film over the semiconductor film;forming a second insulating film over the first conductive film; forminga third insulating film over the second insulating film; forming a firstmask over the third insulating film; partly etching the third insulatingfilm, the second insulating film, the first conductive film, thesemiconductor film, and the first oxide film with the first mask to forma first oxide layer, a semiconductor layer, a second insulating layer,and a third insulating layer in island shapes; forming a fourthinsulating film over the first insulating layer and the third insulatinglayer; performing a chemical mechanical polishing process on the fourthinsulating film until the third insulating layer is exposed to form afourth insulating layer; forming a second mask over the fourthinsulating layer and the third insulating layer; forming a sourceelectrode layer, a drain electrode layer, a fifth insulating layer, anda sixth insulating layer with the second mask; forming a second oxidefilm over the fourth insulating layer, the sixth insulating layer, andthe semiconductor layer; forming a fifth insulating film over the secondoxide film; forming a second conductive film over the fifth insulatingfilm; and performing a chemical mechanical polishing process on thesecond conductive film, the fifth insulating film, and the second oxidefilm to form a second oxide layer, a gate insulating layer, and a gateelectrode layer.

It is preferable that first heat treatment be performed after thesemiconductor film is formed, that a mixed layer of the secondinsulating film and the third insulating film be formed when the thirdinsulating film is formed and, at the same time, oxygen is added to themixed layer or the second insulating film, that a seventh insulatinglayer be formed over the fourth insulating layer, the sixth insulatinglayer, the second oxide layer, the gate insulating layer, and the gateelectrode layer, that a mixed layer of the fourth insulating layer andthe seventh insulating layer be formed when the seventh insulating layeris formed and, at the same time, oxygen is added to the mixed layer orthe fourth insulating layer, and that second heat treatment be performedto diffuse the oxygen to the semiconductor layer.

It is preferable that each of the second insulating film and the fourthinsulating film be an insulating film containing oxygen, and that eachof the third insulating film and the seventh insulating layer be formedby a sputtering method with an oxygen gas.

It is preferable that each of the second insulating film and the fourthinsulating film be a silicon oxide film, and that each of the thirdinsulating film and the seventh insulating layer be formed by asputtering method with an oxygen gas of 50 vol % or higher and analuminum oxide target.

It is preferable that the second heat treatment be performed at atemperature higher than or equal to 300° C. and lower than or equal to450° C.

Any of the aforementioned semiconductor devices can be combined with amicrophone, a speaker, and a housing.

According to one embodiment of the present invention, the parasiticcapacitance in the vicinity of a transistor can be reduced.Alternatively, a semiconductor device with favorable electricalcharacteristics can be provided. Alternatively, a semiconductor devicewith high reliability can be provided. Alternatively, variations incharacteristics caused by a manufacturing process of a transistor or asemiconductor device can be reduced. Alternatively, a manufacturingprocess of a transistor can be stabilized. Alternatively, asemiconductor device including an oxide semiconductor layer having fewoxygen vacancies can be provided. Alternatively, a semiconductor devicethat can be manufactured in a simple process can be provided.Alternatively, a semiconductor device with a structure in which thedensity of interface states in the vicinity of the oxide semiconductorlayer can be reduced can be provided. A semiconductor device with lowpower consumption can be provided. A novel semiconductor device or thelike can be provided. A method for manufacturing the semiconductordevice can be provided.

Note that the descriptions of these effects do not disturb the existenceof other effects. In one embodiment of the present invention, there isno need to achieve all the effects. Other effects will be apparent fromand can be derived from the description of the specification, thedrawings, the claims, and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1C are a top view and cross-sectional views illustrating atransistor.

FIGS. 2A to 2D are cross-sectional views each illustrating a transistor.

FIGS. 3A and 3B are an enlarged cross-sectional view and a band diagramof a transistor.

FIGS. 4A to 4D illustrate ALD deposition mechanism.

FIGS. 5A and 5B are schematic views of an ALD apparatus.

FIGS. 6A to 6C are a top view and cross-sectional views illustrating amethod for manufacturing a transistor.

FIGS. 7A to 7C are a top view and cross-sectional views illustrating themethod for manufacturing a transistor.

FIGS. 8A to 8C are a top view and cross-sectional views illustrating themethod for manufacturing a transistor.

FIGS. 9A to 9C are a top view and cross-sectional views illustrating themethod for manufacturing a transistor.

FIGS. 10A to 10C are a top view and cross-sectional views illustratingthe method for manufacturing a transistor.

FIGS. 11A to 11C are a top view and cross-sectional views illustratingthe method for manufacturing a transistor.

FIGS. 12A to 12C are a top view and cross-sectional views illustratingthe method for manufacturing a transistor.

FIGS. 13A to 13C are a top view and cross-sectional views illustratingthe method for manufacturing a transistor.

FIGS. 14A to 14C are a top view and cross-sectional views illustratingthe method for manufacturing a transistor.

FIGS. 15A to 15C are a top view and cross-sectional views of atransistor.

FIGS. 16A to 16C are a top view and cross-sectional views illustrating amethod for manufacturing a transistor.

FIGS. 17A to 17C are a top view and cross-sectional views illustratingthe method for manufacturing a transistor.

FIGS. 18A to 18C are a top view and cross-sectional views illustratingthe method for manufacturing a transistor.

FIGS. 19A to 19C are a top view and cross-sectional views illustratingthe method for manufacturing a transistor.

FIGS. 20A to 20C are a top view and cross-sectional views illustratingthe method for manufacturing a transistor.

FIGS. 21A to 21D are cross-sectional views illustrating the method formanufacturing a transistor.

FIGS. 22A to 22D are cross-sectional views illustrating the method formanufacturing a transistor.

FIGS. 23A to 23C are a top view and cross-sectional views illustratingthe method for manufacturing a transistor.

FIGS. 24A to 24C are a top view and cross-sectional views of atransistor.

FIGS. 25A to 25C are a top view and cross-sectional views illustrating amethod for manufacturing a transistor.

FIGS. 26A to 26C are a top view and cross-sectional views illustratingthe method for manufacturing a transistor.

FIGS. 27A to 27C are a top view and cross-sectional views illustratingthe method for manufacturing a transistor.

FIGS. 28A to 28C are a top view and cross-sectional views illustratingthe method for manufacturing a transistor.

FIGS. 29A to 29C are a top view and cross-sectional views illustratingthe method for manufacturing a transistor.

FIGS. 30A to 30C are a top view and cross-sectional views illustratingthe method for manufacturing a transistor.

FIGS. 31A to 31C are a top view and cross-sectional views illustratingthe method for manufacturing a transistor.

FIGS. 32A to 32C are a top view and cross-sectional views illustratingthe method for manufacturing a transistor.

FIGS. 33A to 33D are Cs-corrected high-resolution TEM images of a crosssection of a CAAC-OS and a cross-sectional schematic view of theCAAC-OS.

FIGS. 34A to 34D are Cs-corrected high-resolution TEM images of a planeof a CAAC-OS.

FIGS. 35A to 35C show structural analysis of a CAAC-OS and a singlecrystal oxide semiconductor by XRD.

FIGS. 36A and 36B show electron diffraction patterns of a CAAC-OS.

FIG. 37 shows a change of crystal parts of an In—Ga—Zn oxide owing toelectron irradiation.

FIGS. 38A to 38D are cross-sectional views and circuit diagrams of asemiconductor device.

FIGS. 39A to 39C are a cross-sectional view and circuit diagrams of asemiconductor device.

FIGS. 40A and 40B are plan views of an imaging device.

FIGS. 41A and 41B are plan views of pixels of an imaging device.

FIGS. 42A and 42B are cross-sectional views of an imaging device.

FIGS. 43A and 43B are cross-sectional views of an imaging device.

FIG. 44 illustrates a configuration example of an RF tag.

FIG. 45 illustrates a configuration example of a CPU.

FIG. 46 is a circuit diagram of a memory element.

FIGS. 47A to 47C illustrate a configuration example of a display deviceand circuit diagrams of pixels.

FIGS. 48A and 48B are a top view and a cross-sectional view illustratinga display device.

FIGS. 49A and 49B are a top view and a cross-sectional view illustratinga display device.

FIG. 50 illustrates a display module.

FIG. 51A is a perspective view illustrating a cross-sectional structureof a package using a lead frame interposer, and FIG. 51B illustrates astructure of a module.

FIGS. 52A to 52E illustrate electronic devices.

FIGS. 53A to 53D illustrate electronic devices.

FIGS. 54A to 54C illustrate electronic devices.

FIGS. 55A to 55F illustrate electronic devices.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments will be described in detail with reference to the drawings.Note that the present invention is not limited to the followingdescription and it will be readily appreciated by those skilled in theart that modes and details can be modified in various ways withoutdeparting from the spirit and the scope of the present invention.Therefore, the present invention should not be limited to thedescriptions of the embodiments below. Note that in structures of thepresent invention described below, the same portions or portions havingsimilar functions are denoted by the same reference numerals indifferent drawings, and description thereof is not repeated in somecases. It is also to be noted that the same components are denoted bydifferent hatching patterns in different drawings, or the hatchingpatterns are omitted in some cases.

For example, in this specification and the like, an explicit description“X and Y are connected” means that X and Y are electrically connected, Xand Y are functionally connected, and X and Y are directly connected.Accordingly, another element may be provided between elements having aconnection relation illustrated in drawings and texts, without beinglimited to a predetermined connection relation, for example, theconnection relation illustrated in the drawings and the texts.

Here, X and Y each denote an object (e.g., a device, an element, acircuit, a wiring, an electrode, a terminal, a conductive film, a layer,or the like).

Examples of the case where X and Y are directly connected include thecase where an element that allows an electrical connection between X andY (e.g., a switch, a transistor, a capacitor, an inductor, a resistor, adiode, a display element, a light-emitting element, and a load) is notconnected between X and Y, and the case where X and Y are connectedwithout the element that allows the electrical connection between X andY provided therebetween.

For example, in the case where X and Y are electrically connected, oneor more elements that enable electrical connection between X and Y(e.g., a switch, a transistor, a capacitor, an inductor, a resistor, adiode, a display element, a light-emitting element, or a load) can beconnected between X and Y. A switch is controlled to be on or off. Thatis, a switch is conducting or not conducting (is turned on or off) todetermine whether current flows therethrough or not. Alternatively, theswitch has a function of selecting and changing a current path. Notethat the case where X and Y are electrically connected includes the casewhere X and Y are directly connected.

For example, in the case where X and Y are functionally connected, oneor more circuits that enable functional connection between X and Y(e.g., a logic circuit such as an inverter, a NAND circuit, or a NORcircuit; a signal converter circuit such as a DA converter circuit, anAD converter circuit, or a gamma correction circuit; a potential levelconverter circuit such as a power supply circuit (e.g., a step-up dc-dcconverter, or a step-down dc-dc converter) or a level shifter circuitfor changing the potential level of a signal; a voltage source; acurrent source; a switching circuit; an amplifier circuit such as acircuit that can increase signal amplitude, the amount of current, orthe like, an operational amplifier, a differential amplifier circuit, asource follower circuit, or a buffer circuit; a signal generationcircuit; a memory circuit; and/or a control circuit) can be connectedbetween X and Y. Note that for example, in the case where a signaloutput from X is transmitted to Y even when another circuit isinterposed between X and Y, X and Y are functionally connected. Notethat the case where X and Y are functionally connected includes the casewhere X and Y are directly connected and the case where X and Y areelectrically connected.

Note that in this specification and the like, an explicit description “Xand Y are electrically connected” means that X and Y are electricallyconnected (i.e., the case where X and Y are connected with anotherelement or another circuit provided therebetween), X and Y arefunctionally connected (i.e., the case where X and Y are functionallyconnected with another circuit provided therebetween), and X and Y aredirectly connected (i.e., the case where X and Y are connected withoutanother element or another circuit provided therebetween). That is, inthis specification and the like, the explicit description “X and Y areelectrically connected” is the same as the description “X and Y areconnected”.

Note that, for example, the case where a source (or a first terminal orthe like) of a transistor is electrically connected to X through (or notthrough) Z1 and a drain (or a second terminal or the like) of thetransistor is electrically connected to Y through (or not through) Z2,or the case where a source (or a first terminal or the like) of atransistor is directly connected to one part of Z1 and another part ofZ1 is directly connected to X while a drain (or a second terminal or thelike) of the transistor is directly connected to one part of Z2 andanother part of Z2 is directly connected to Y, can be expressed by usingany of the following expressions.

The expressions include, for example, “X, Y, a source (or a firstterminal or the like) of a transistor, and a drain (or a second terminalor the like) of the transistor are electrically connected to each other,and X, the source (or the first terminal or the like) of the transistor,the drain (or the second terminal or the like) of the transistor, and Yare electrically connected to each other in this order”, “a source (or afirst terminal or the like) of a transistor is electrically connected toX, a drain (or a second terminal or the like) of the transistor iselectrically connected to Y, and X, the source (or the first terminal orthe like) of the transistor, the drain (or the second terminal or thelike) of the transistor, and Y are electrically connected to each otherin this order”, and “X is electrically connected to Y through a source(or a first terminal or the like) and a drain (or a second terminal orthe like) of a transistor, and X, the source (or the first terminal orthe like) of the transistor, the drain (or the second terminal or thelike) of the transistor, and Y are provided to be connected in thisorder”. When the connection order in a circuit configuration is definedby an expression similar to the above examples, a source (or a firstterminal or the like) and a drain (or a second terminal or the like) ofa transistor can be distinguished from each other to specify thetechnical scope.

Other examples of the expressions include, “a source (or a firstterminal or the like) of a transistor is electrically connected to Xthrough at least a first connection path, the first connection path doesnot include a second connection path, the second connection path is apath between the source (or the first terminal or the like) of thetransistor and a drain (or a second terminal or the like) of thetransistor, Z1 is on the first connection path, the drain (or the secondterminal or the like) of the transistor is electrically connected to Ythrough at least a third connection path, the third connection path doesnot include the second connection path, and Z2 is on the thirdconnection path” and “a source (or a first terminal or the like) of atransistor is electrically connected to X at least with a firstconnection path through Z1, the first connection path does not include asecond connection path, the second connection path includes a connectionpath through which the transistor is provided, a drain (or a secondterminal or the like) of the transistor is electrically connected to Yat least with a third connection path through Z2, and the thirdconnection path does not include the second connection path”. Stillanother example of the expression is “a source (or a first terminal orthe like) of a transistor is electrically connected to X through atleast Z1 on a first electrical path, the first electrical path does notinclude a second electrical path, the second electrical path is anelectrical path from the source (or the first terminal or the like) ofthe transistor to a drain (or a second terminal or the like) of thetransistor, the drain (or the second terminal or the like) of thetransistor is electrically connected to Y through at least Z2 on a thirdelectrical path, the third electrical path does not include a fourthelectrical path, and the fourth electrical path is an electrical pathfrom the drain (or the second terminal or the like) of the transistor tothe source (or the first terminal or the like) of the transistor”. Whenthe connection path in a circuit configuration is defined by anexpression similar to the above examples, a source (or a first terminalor the like) and a drain (or a second terminal or the like) of atransistor can be distinguished from each other to specify the technicalscope.

Note that these expressions are examples and there is no limitation onthe expressions. Here, X, Y, Z1, and Z2 each denote an object (e.g., adevice, an element, a circuit, a wiring, an electrode, a terminal, aconductive film, a layer, or the like).

Even when independent components are electrically connected to eachother in a circuit diagram, one component has functions of a pluralityof components in some cases. For example, when part of a wiring alsofunctions as an electrode, one conductive film functions as the wiringand the electrode. Thus, “electrical connection” in this specificationincludes in its category such a case where one conductive film hasfunctions of a plurality of components.

<Notes on the Description for Drawings>

In this specification, terms for describing arrangement, such as “over”,“above”, “under”, and “below”, are used for convenience in describing apositional relation between components with reference to drawings.Further, the positional relation between components is changed asappropriate in accordance with a direction in which each component isdescribed. Thus, the positional relation is not limited to thatdescribed with a term used in this specification and can be explainedwith another term as appropriate depending on the situation.

The term “over” or “under” does not necessarily mean that a component isplaced “directly above and in contact with” or “directly below and incontact with” another component. For example, the expression “electrodeB over insulating layer A” does not necessarily mean that the electrodeB is on and in direct contact with the insulating layer A and can meanthe case where another component is provided between the insulatinglayer A and the electrode B.

In this specification, the term “parallel” indicates that the angleformed between two straight lines is greater than or equal to −10° andless than or equal to 10°, and accordingly also includes the case wherethe angle is greater than or equal to −5° and less than or equal to 5°.The term “substantially parallel” indicates that the angle formedbetween two straight lines is greater than or equal to −30° and lessthan or equal to 30°. The term “perpendicular” indicates that the angleformed between two straight lines is greater than or equal to 80° andless than or equal to 100°, and accordingly also includes the case wherethe angle is greater than or equal to 85° and less than or equal to 95°.The term “substantially perpendicular” indicates that the angle formedbetween two straight lines is greater than or equal to 60° and less thanor equal to 120°.

In this specification, trigonal and rhombohedral crystal systems areincluded in a hexagonal crystal system.

In drawings, the size, the layer thickness, or the region is determinedarbitrarily for description convenience. Therefore, embodiments of thepresent invention are not limited to such a scale. Note that thedrawings are schematically illustrated for clarity, and embodiments ofthe present invention are not limited to shapes or values shown in thedrawings.

In drawings such as a top view (also referred to as plan view or layoutview) and a perspective view, some of components might not beillustrated for clarity of the drawings.

The expression “being the same” may refer to having the same area or thesame shape or may refer to being at the same height in a perpendiculardirection from a substrate surface. The same height can be restated asthe same plane. In addition, the expression “being the same” include acase of “being substantially the same” because a manufacturing processmight cause some differences in shape, plane, or height.

<Notes on Expressions that can be Rephrased>

In this specification or the like, in describing connections of atransistor, one of a source and a drain is referred to as “one of asource and a drain” (or a first electrode or a first terminal), and theother of the source and the drain is referred to as “the other of thesource and the drain” (or a second electrode or a second terminal). Thisis because a source and a drain of a transistor are interchangeabledepending on the structure, operation conditions, or the like of thetransistor. Note that the source or the drain of the transistor can alsobe referred to as a source (or drain) terminal, a source (or drain)electrode, or the like as appropriate depending on the situation.

In addition, in this specification and the like, the term such as an“electrode” or a “wiring” does not limit a function of the component.For example, an “electrode” is used as part of a “wiring” in some cases,and vice versa. Further, the term “electrode” or “wiring” can also meana combination of a plurality of “electrodes” and “wirings” formed in anintegrated manner.

In this specification and the like, a transistor is an element having atleast three terminals of a gate, a drain, and a source. In addition, thetransistor has a channel region between a drain (a drain terminal, adrain region, or a drain electrode) and a source (a source terminal, asource region, or a source electrode), and current can flow through thedrain, the channel region, and the source.

Since the source and the drain of the transistor change depending on thestructure, operating conditions, and the like of the transistor, it isdifficult to define which is a source or a drain. Thus, a portion thatfunctions as a source or a portion that functions as a drain is notreferred to as a source or a drain in some cases. In that case, one ofthe source and the drain might be referred to as a first electrode, andthe other of the source and the drain might be referred to as a secondelectrode.

In this specification, ordinal numbers such as “first”, “second”, and“third” are used to avoid confusion among components, and thus do notlimit the number of the components.

In this specification and the like, a structure in which a flexibleprinted circuit (FPC), a tape carrier package (TCP), or the like isattached to a substrate of a display panel, or a structure in which anintegrated circuit (IC) is directly mounted on a substrate by a chip onglass (COG) method is referred to as a display device in some cases.

Note that the terms “film” and “layer” can be interchanged with eachother depending on the case or circumstances. For example, the term“conductive layer” can be changed into the term “conductive film” insome cases. In addition, the term “insulating film” can be changed intothe term “insulating layer” in some cases.

<Notes on Definitions of Terms>

The following are definitions of the terms in this specification and thelike.

In this specification, the term “trench” or “groove” refers to adepression with a narrow belt shape.

In this specification, a silicon oxynitride film is described asSiO_(x)N_(y) in some cases, for example. At this time, x and y may benatural numbers or decimals.

<<Connection>>

In this specification, when it is described that “A and B are connectedto each other”, the case where A and B are electrically connected toeach other is included in addition to the case where A and B aredirectly connected to each other. Here, the expression “A and B areelectrically connected” means the case where electric signals can betransmitted and received between A and B when an object having anyelectric action exists between A and B.

Note that a content (or may be part of the content) described in oneembodiment may be applied to, combined with, or replaced by a differentcontent (or may be part of the different content) described in theembodiment and/or a content (or may be part of the content) described inone or a plurality of different embodiments.

Note that in each embodiment, a content described in the embodiment is acontent described with reference to a variety of diagrams or a contentdescribed with a text described in this specification.

Note that by combining a diagram (or may be part of the diagram)illustrated in one embodiment with another part of the diagram, adifferent diagram (or may be part of the different diagram) illustratedin the embodiment, and/or a diagram (or may be part of the diagram)illustrated in one or a plurality of different embodiments, much morediagrams can be formed.

Embodiment 1

In this embodiment, a semiconductor device which is one embodiment ofthe present invention and a manufacturing method thereof will bedescribed with reference to drawings.

FIGS. 1A to 1C are a top view and cross-sectional views of a transistor10 of one embodiment of the present invention. FIG. 1A is a top view,FIG. 1B is a cross-sectional view taken along the dashed-dotted lineA1-A2 in FIG. 1A, and FIG. 1C is a cross-sectional view taken along thedashed-dotted line A3-4 in FIG. 1A. Note that in FIG. 1A, somecomponents are scaled up or down in size or omitted for easyunderstanding. The directions of the dashed-dotted line A1-A2 and thedashed-dotted line A3-A4 can be referred to as a channel lengthdirection and a channel width direction, respectively, in some cases.

The transistor 10 includes a substrate 100, an insulating layer 110, aninsulator 121, a semiconductor layer 122, an insulator 123, a sourceelectrode layer 130, a drain electrode layer 140, a gate insulatinglayer 150, a gate electrode layer 160, an insulating layer 175, aninsulating layer 173, and an insulating layer 170. The transistor 10includes the insulating layer 110 over the substrate 100. The transistor10 includes the insulator 121 over the insulating layer 110. Thetransistor 10 includes the semiconductor layer 122 over the insulator121. The transistor 10 includes the source electrode layer 130 and thedrain electrode layer 140 over the semiconductor layer 122, and thesource electrode layer 130 and the drain electrode layer 140 areelectrically connected to the semiconductor layer 122. The transistor 10includes the insulating layer 173, and the insulating layer 173 includesregions in contact with side surfaces of the insulator 121, thesemiconductor layer 122, the insulator 123, the source electrode layer130, and the drain electrode layer 140. The transistor 10 includes theinsulating layer 175 over the insulating layer 173, the source electrodelayer 130, and the drain electrode layer 140, and the insulating layer175 includes a region in contact with a side surface of the insulator123. The transistor 10 includes the insulator 123 over the semiconductorlayer 122, and the insulator 123 includes regions in contact with sidesurfaces of the insulator 121, the insulating layer 173, and theinsulating layer 175 as illustrated in FIG. 1C. The insulator 123includes regions in contact with a bottom surface of the insulatinglayer 170 and side surfaces of the insulating layer 175, the sourceelectrode layer 130, and the drain electrode layer 140 as illustrated inFIG. 1B. The transistor 10 includes the gate insulating layer 150 overthe insulator 123. The transistor 10 includes the gate electrode layer160 over the gate insulating layer 150.

<Insulator>

An insulator (e.g., the insulators 121 and 123) refers to a layer whichbasically has an insulating property and in which current can flowthrough the interface with the semiconductor layer and the vicinitythereof when a gate electric field or a drain electric field isincreased.

The structure described above is characterized by its high heatdissipation effect: heat generated by the operation of the transistor 10in the insulator 121, the semiconductor layer 122, and the insulator 123can be sufficiently released because the semiconductor layer 122 and theinsulator 123 each include regions in contact with the source electrodelayer 130 and the drain electrode layer 140.

When the insulating layer 170 is formed, in the transistor 10, a mixedlayer including materials of the insulating layers 175 and 170, a gasused during formation of the insulating layer 170, and the like isformed at the interface with the insulating layer 175, and oxygen(excess oxygen (exO)) is added to the mixed layer or the insulatinglayer 175. With heat treatment, the oxygen can be further diffused tothe semiconductor layer 122, and oxygen vacancies that exist in theinsulator 121 and the semiconductor layer 122 can be filled with theoxygen. Thus, transistor characteristics (e.g., a threshold voltage orreliability) can be improved.

The excess oxygen that is added during the formation of the insulatinglayer 170 exists in a variety of states such as an oxygen radical, anoxygen ion, or an oxygen atom during the formation by a sputteringmethod owing to the influence of applied voltage, power, plasma,substrate temperature, or the like. At this time, the excess oxygen isin a state of having higher energy than a stable state and thus can betaken into the insulating layer 175.

Note that the method of adding oxygen is not limited to the abovemethod, and the insulating layer 110 may contain the excess oxygenduring formation, or another method (e.g., an ion implantation method oran ion plasma immersion method) may be employed after the formation.

In the channel width direction of the transistor 10, the gate electrodelayer 160 faces the side surfaces of the insulator 121, thesemiconductor layer 122, and the insulator 123 with the gate insulatinglayer 150 therebetween as illustrated in the cross-sectional view ofFIG. 1C taken along the dashed-dotted line A3-4. In other words, theinsulator 121, the semiconductor layer 122, and the insulator 123 aresurrounded by electric field of the gate electrode layer 160 in thechannel width direction when voltage is applied to the gate electrodelayer 160. The transistor structure in which the semiconductor layer 122is surrounded by electric field of the gate electrode layer 160 isreferred to as a surrounded channel (s-channel) structure. Furthermore,the gate electrode layer, the source electrode layer, and the drainelectrode layer of the transistor 10 can be formed with a groove in aself-aligned manner; thus, alignment accuracy can be improved andminiaturized transistors can be easily manufactured. Note that such astructure described in this embodiment is referred to as a self-aligned(SA) s-channel FET structure, a trench-gate s-channel FET structure, atrench-gate self-aligned (TGSA) s-channel FET structure, or a gate-lastself-aligned (GLSA) s-channel FET structure.

The insulator 121, the semiconductor layer 122, and the insulator 123are collectively referred to as a semiconductor layer 120. When atransistor having the TGSA structure is in the on state, a channel isformed in the entire semiconductor layer 120 (bulk), so that theon-state current is increased. When the transistor having the TGSAstructure is in the off state, the entire channel region formed in thesemiconductor layer 120 can be depleted; thus, the off-state current canbe further reduced.

In addition, the transistor 10 has the TGSA structure, whereby parasiticcapacitance generated between the gate electrode and the sourceelectrode or between the gate electrode and the drain electrode isreduced, and the cut-off frequency characteristics of the transistor 10are improved. That is, high-speed response of the transistor 10 can beachieved.

As will be described later in this embodiment, after planarizationtreatment is performed on a second insulating film to be the insulatinglayer 175 until the source electrode layer 130 and the drain electrodelayer 140 are exposed, a third insulating film to be the insulatinglayer 173 is formed. At this time, it is preferable that the topsurfaces of the insulating layer 173, the source electrode layer 130,and the drain electrode layer 140 be at the same level and parallel to asubstrate surface. Thus, the insulating layer 173 over the sourceelectrode layer 130 and the drain electrode layer 140 can be formeduniformly in the substrate surface; therefore, a step of forming agroove portion 174 (e.g., time for etching treatment) can be stabilized.Eventually, the transistor 10 can be manufactured stably. Accordingly,the transistor can have a stable shape, which suppresses fluctuation oftransistor characteristics.

Note that the top surface of the source electrode layer 130 or the drainelectrode layer 140 may be located under, over, or at the same level asthe bottom surface of the gate electrode layer 160.

Alternatively, in the transistor 10, the top surface of the gateelectrode layer 160 may be located under the top surface of theinsulating layer 175. Alternatively, the source electrode layer 130 andthe drain electrode layer 140 may be shorter or longer than thesemiconductor layer 122 in the channel length direction.

<Channel Length>

Note that the channel length refers to, for example, a distance betweena source (a source region or a source electrode) and a drain (a drainregion or a drain electrode) in a region where a semiconductor (or aportion where a current flows in a semiconductor when a transistor ison) and a gate electrode overlap with each other or a region where achannel is formed, in a top view of the transistor. In one transistor,channel lengths in all regions are not necessarily the same. In otherwords, the channel length of one transistor is not fixed to one value insome cases. Therefore, in this specification, the channel length is anyone of values, the maximum value, the minimum value, or the averagevalue in a region where a channel is formed.

<Channel Width>

Note that the channel width refers to, for example, the length of aregion where a semiconductor (or a portion where a current flows in asemiconductor when a transistor is on) and a gate electrode overlap witheach other. In one transistor, channel widths in all regions are notnecessarily the same value. In other words, the channel width of onetransistor is not fixed to one value in some cases. Therefore, in thisspecification, the channel width is any one of values, the maximumvalue, the minimum value, or the average value in a region where achannel is formed.

Note that depending on transistor structures, a channel width in aregion where a channel is actually formed (hereinafter referred to as aneffective channel width) is different from a channel width shown in atop view of a transistor (hereinafter referred to as an apparent channelwidth) in some cases. For example, in a transistor having athree-dimensional structure, an effective channel width is greater thanan apparent channel width shown in a top view of the transistor, and itsinfluence cannot be ignored in some cases. For example, in aminiaturized transistor having a three-dimensional channel, theproportion of a channel region formed in a side surface of asemiconductor is high in some cases. In that case, an effective channelwidth obtained when a channel is actually formed is greater than anapparent channel width shown in the top view.

In a transistor having a three-dimensional structure, an effectivechannel width is difficult to measure in some cases. For example,estimation of an effective channel width from a design value requires anassumption that the shape of a semiconductor is known. Therefore,without accurate information on the shape of a semiconductor, it isdifficult to measure an effective channel width accurately.

<SCW>

Therefore, in this specification, in a top view of a transistor, anapparent channel width in a region where a semiconductor and a gateelectrode overlap with each other is referred to as a surrounded channelwidth (SCW) in some cases. Further, in this specification, in the casewhere the term “channel width” is simply used, it may denote asurrounded channel width or an apparent channel width. Alternatively, inthis specification, in the case where the term “channel width” is simplyused, it may denote an effective channel width in some cases. Note thatthe values of a channel length, a channel width, an effective channelwidth, an apparent channel width, a surrounded channel width, and thelike can be determined by obtaining and analyzing a cross-sectional TEMimage and the like.

Note that in the case where field-effect mobility, a current value perchannel width, and the like of a transistor are obtained by calculation,a surrounded channel width may be used for the calculation. In thatcase, a value different from the value obtained by calculation using aneffective channel width is obtained in some cases.

<Improvement of Characteristics in Miniaturization>

High integration of a semiconductor device requires miniaturization of atransistor. However, it is known that miniaturization of a transistorcauses deterioration of the electrical characteristics of thetransistor. A decrease in channel width causes a reduction in on-statecurrent.

However, in the transistor of one embodiment of the present inventionshown in FIGS. 1A to 1C, as described above, the insulator 123 is formedso as to cover the semiconductor layer 122 where a channel is formed anda channel formation layer and the gate insulating film are not incontact with each other. Accordingly, scattering of carriers at theinterface between the channel formation layer and the gate insulatinglayer can be reduced and the on-state current of the transistor can beincreased.

In the transistor of one embodiment of the present invention, the gateelectrode layer 160 is formed to electrically surround the semiconductorlayer 122, which is to be a channel, in the channel width direction;accordingly, a gate electric field is applied to the semiconductor layer122 in the side surface direction in addition to the perpendiculardirection. In other words, a gate electric field is applied to thesemiconductor layer 122 entirely, so that current flows in the whole ofthe semiconductor layer 122, leading to a further increase in on-statecurrent.

In the transistor of one embodiment of the present invention, theinsulator 123 is formed over the insulator 121 and the semiconductorlayer 122, so that an interface state is less likely to be formed. Inaddition, impurities do not enter the semiconductor layer 122 from aboveand below because the semiconductor layer 122 is positioned at themiddle. Therefore, the transistor can achieve not only the increase inthe on-state current of the transistor but also stabilization of thethreshold voltage and a reduction in the S value (subthreshold swing).Thus, I_(cut) (current when gate voltage VG is 0 V) can be reduced andpower consumption can be reduced. Further, since the threshold voltageof the transistor becomes stable, long-term reliability of thesemiconductor device can be improved.

Although an example where a channel or the like is formed in thesemiconductor layer 120 (the semiconductor layer 122) or the like isdescribed in this embodiment, one embodiment of the present invention isnot limited thereto. For example, depending on cases or conditions, achannel, the vicinity of the channel, a source region, a drain region,or the like may be formed using a material containing silicon (includingstrained silicon), germanium, silicon germanium, silicon carbide,gallium arsenide, aluminum gallium arsenide, indium phosphide, galliumnitride, an organic semiconductor, or the like.

<Structure of Transistor>

A structure of a transistor of one embodiment of the present inventionis described.

<<Substrate 100>>

A glass substrate, a ceramic substrate, a quartz substrate, a sapphiresubstrate, or the like can be used as the substrate 100. Alternatively,a single crystal semiconductor substrate or a polycrystallinesemiconductor substrate of silicon or silicon carbide, a compoundsemiconductor substrate of silicon germanium, a silicon-on-insulator(SOI) substrate, or the like may be used. Still alternatively, any ofthese substrates provided with a semiconductor element may be used. Thesubstrate 100 is not limited to a simple supporting substrate, and maybe a substrate where a device such as a transistor is formed. In thatcase, at least one of the gate electrode layer 160, the source electrodelayer 130, and the drain electrode layer 140 of the transistor may beelectrically connected to above device.

Alternatively, a flexible substrate may be used as the substrate 100. Asa method for providing the transistor over a flexible substrate, thereis a method in which the transistor is formed over a non-flexiblesubstrate and then the transistor is separated and transferred to thesubstrate 100 which is a flexible substrate. In that case, a separationlayer is preferably provided between the non-flexible substrate and thetransistor. As the substrate 100, a sheet, a film, or a foil containinga fiber may be used. The substrate 100 may have elasticity. Thesubstrate 100 may have a property of returning to its original shapewhen bending or pulling is stopped. Alternatively, the substrate 100 mayhave a property of not returning to its original shape. The substrate100 has a thickness of, for example, greater than or equal to 5 μm andless than or equal to 700 μm, preferably greater than or equal to 10 μmand less than or equal to 500 μm and further preferably greater than orequal to 15 μm and less than or equal to 300 μm. When the substrate 100has a small thickness, the weight of the semiconductor device can bereduced. When the substrate 100 has a small thickness, even in the caseof using glass or the like, the substrate 100 may have elasticity or aproperty of returning to its original shape when bending or pulling isstopped. Therefore, an impact applied to the semiconductor device overthe substrate 100, which is caused by dropping or the like, can bereduced. That is, a durable semiconductor device can be provided.

For the substrate 100 which is a flexible substrate, metal, an alloy,resin, glass, or fiber thereof can be used, for example. The flexiblesubstrate 100 preferably has a lower coefficient of linear expansionbecause deformation due to an environment is suppressed. The flexiblesubstrate 100 is formed using, for example, a material whose coefficientof linear expansion is lower than or equal to 1×10⁻³/K, lower than orequal to 5×10⁻⁵/K, or lower than or equal to 1×10⁻⁵/K. Examples of theresin include polyester, polyolefin, polyamide (e.g., nylon or aramid),polyimide, polycarbonate, acrylic, and polytetrafluoroethylene (PTFE).In particular, aramid is preferably used for the flexible substrate 100because of its low coefficient of linear expansion.

<<Insulating Layer 110>>

The insulating layer 110 can have a function of supplying oxygen to thesemiconductor layer 120 (the semiconductor layer 122) as well as afunction of preventing diffusion of impurities from the substrate 100.For this reason, the insulating layer 110 is preferably an insulatingfilm containing oxygen and further preferably an insulating filmcontaining oxygen in which the oxygen content is higher than that in thestoichiometric composition. The insulating layer 110 is a film of whichthe amount of released oxygen when converted into oxygen atoms is1.0×10¹⁹ atoms/cm³ or more in TDS analysis. Note that the temperature ofthe film surface in the TDS analysis is preferably higher than or equalto 100° C. and lower than or equal to 700° C., or higher than or equalto 100° C. and lower than or equal to 500° C. In the case where thesubstrate 100 is provided with another device as described above, theinsulating layer 110 also has a function as an interlayer insulatingfilm. In that case, the insulating layer 110 is preferably subjected toplanarization treatment such as chemical mechanical polishing (CMP)treatment so as to have a flat surface.

<<Insulators 121 and 123 and Semiconductor Layer 122>>

An oxide that can be used for each of the insulator 121, thesemiconductor layer 122, and the insulator 123 preferably contains atleast indium (In) or zinc (Zn). Alternatively, both In and Zn arepreferably contained. In order to reduce fluctuations in the electricalcharacteristics of the transistors including the oxides assemiconductors, the oxides preferably contain a stabilizer in additionto In and Zn. Typically, In—Ga oxide, In—Zn oxide, In—Mg oxide, Zn—Mgoxide, or In-M-Zn oxide (M is Al, Ti, Ga, Y, Zr, Sn, La, Ce, Mg, or Nd)can be given as the oxide.

As examples of a stabilizer, gallium (Ga), tin (Sn), hafnium (Hf),aluminum (Al), zirconium (Zr), and the like can be given. As anotherexample of the stabilizer, lanthanoid such as lanthanum (La), cerium(Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm), europium (Eu),gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium(Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu) can be given.

Note that in the case where the insulator 123 is an In-M-Zn oxide, theproportions of In and M when the summation of In and M is assumed to be100 atomic % are preferably as follows: the atomic percentage of In isgreater than or equal to 25 atomic % and the atomic percentage of M isless than 75 atomic %, preferably the atomic percentage of In is greaterthan or equal to 34 atomic % and the atomic percentage of M is less than66 atomic %.

The indium and gallium contents in the insulator 123 can be comparedwith each other by time-of-flight secondary ion mass spectrometry(TOF-SIMS), X-ray photoelectron spectrometry (XPS), or inductivelycoupled plasma mass spectrometry (ICP-MS).

Since the semiconductor layer 122 has an energy gap of 2 eV or more,preferably 2.5 eV or more and further preferably 3 eV or more, theoff-state current of the transistor 10 can be low.

The thickness of the semiconductor layer 122 is greater than or equal to3 nm and less than or equal to 200 nm, preferably greater than or equalto 3 nm and less than or equal to 100 nm and further preferably greaterthan or equal to 3 nm and less than or equal to 50 nm.

Each of the insulator 121 and the insulator 123 is an oxide filmincluding one or more elements contained in the semiconductor layer 122.Thus, interface scattering is unlikely to occur at the interface betweenthe semiconductor layer 122 and each of the insulator 121 and theinsulator 123. The movement of carriers is not hindered at theinterfaces accordingly, and the transistor 10 can have high field-effectmobility.

Each of the insulator 121 and the insulator 123 is typically In—Gaoxide, In—Zn oxide, In—Mg oxide, Ga—Zn oxide, Zn—Mg oxide, or In-M-Znoxide (M is Al, Ti, Ga, Y, Zr, Sn, La, Ce, Mg, or Nd), and has theenergy level at the conduction band minimum that is closer to a vacuumlevel than that of the semiconductor layer 122 is. Typically, adifference between the energy level at the conduction band minimum ofthe semiconductor layer 122 and the energy level at the conduction bandminimum of each of the insulators 121 and 123 is 0.05 eV or more, 0.07eV or more, 0.1 eV or more, or 0.2 eV or more and also 2 eV or less, 1eV or less, 0.5 eV or less, or 0.4 eV or less. That is, the differencebetween the electron affinity of the semiconductor layer 122 and theelectron affinity of each of the insulators 121 and 123 is 0.05 eV ormore, 0.07 eV or more, 0.1 eV or more, or 0.2 eV or more and also 2 eVor less, 1 eV or less, 0.5 eV or less, or 0.4 eV or less. Note that theelectron affinity refers to a difference between the vacuum level andthe energy level at the conduction band minimum.

When each of the insulator 121 and the insulator 123 contain a largeramount of Al, Ti, Ga, Y, Zr, Sn, La, Ce, Mg, or Nd in an atomic ratiothan the amount of In in an atomic ratio, any of the following effectsmay be obtained.

(1) The energy gap of each of the insulator 121, the semiconductor layer122, and the insulator 123 is widened.

(2) The electron affinity of each of the insulator 121 and the insulator123 is reduced.

(3) Impurities from the outside are blocked.

(4) An insulating property of each of the insulator 121 and theinsulator 123 is higher than that of the semiconductor layer 122.

(5) Oxygen vacancies are less likely to be generated in the insulator121 and the insulator 123 each containing a larger amount of Al, Ti, Ga,Y, Zr, Sn, La, Ce, Mg, or Nd in an atomic ratio than the amount of In inan atomic ratio because Al, Ti, Ga, Y, Zr, Sn, La, Ce, Mg, and Nd is ametal element that can be strongly bonded to oxygen.

Since the insulator 121 and the insulator 123 have higher insulatingproperties than the semiconductor layer 122, they each have a functionsimilar to that of the gate insulating layer.

In the case where each of the insulator 121 and the insulator 123 isIn-M-Zn oxide, the proportion of In and the proportion of M, not takingZn and O into consideration, are preferably less than 50 atomic % andgreater than or equal to 50 atomic %, respectively, and furtherpreferably less than 25 atomic % and greater than or equal to 75 atomic%, respectively.

Further, in the case where each of the insulator 121 and the insulator123 is In-M-Zn oxide (M is Al, Ti, Ga, Y, Zr, Sn, La, Ce, Mg, or Nd),the proportion of M atoms (M is Al, Ti, Ga, Y, Zr, Sn, La, Ce, Mg, orNd) in each of the insulator 121 and the insulator 123 is higher thanthat in the semiconductor layer 122. Typically, the proportion of Matoms in each of the insulator 121 and the insulator 123 is higher thanor equal to 1.5 times, preferably higher than or equal to twice andfurther preferably higher than or equal to three times, as high as thatin the semiconductor layer 122. Any of the above-described elementsrepresented by M is more strongly bonded to oxygen than indium is, andthus has a function of suppressing generation of oxygen vacancies in theinsulator 121 and the insulator 123. That is, oxygen vacancies are lesslikely to be generated in the insulator 121 and the insulator 123 thanin the semiconductor layer 122.

The indium content in the semiconductor layer 122 is preferably higherthan those in the insulator 121 and the insulator 123. As asemiconductor, an s orbital of heavy metal mainly contributes to carriertransfer. When the proportion of In in the semiconductor is increased,overlap of s orbitals is likely to be increased. Therefore, an oxidehaving a composition in which the proportion of In is higher than thatof M has higher mobility than an oxide having a composition in which theproportion of In is equal to or lower than that of M. Thus, with the useof an oxide having a high content of indium for the semiconductor layer122, a transistor having high field-effect mobility can be obtained.

In the case where the semiconductor layer 122 includes In-M-Zn oxide (Mis Al, Ti, Ga, Y, Zr, Sn, La, Ce, Mg, or Nd) and a target having theatomic ratio of metal elements of In:M:Zn=x₁:y₁:z₁ is used for formingthe semiconductor layer 122, x₁/y₁ is preferably greater than or equalto 1/3 and less than or equal to 6 and further preferably greater thanor equal to 1 and less than or equal to 6, and z₁/y₁ is preferablygreater than or equal to 1/3 and less than or equal to 6 and furtherpreferably greater than or equal to 1 and less than or equal to 6. Notethat when z₁/y₁ is greater than or equal to 1 and less than or equal to6, a c-axis aligned crystalline oxide semiconductor (CAAC-OS) film iseasily formed as the semiconductor layer 122. Typical examples of theatomic ratio of metal elements of the target include In:M:Zn=1:1:1,1:1:1.2, 2:1:1.5, 2:1:2.3, 2:1:3, 3:1:2, 4:2:3, and 4:2:4.1.

In the case where the insulator 121 and the insulator 123 includeIn-M-Zn oxide (M is Al, Ti, Ga, Y, Zr, Sn, La, Ce, Mg, or Nd) and atarget having the atomic ratio of metal elements of In:M:Zn=x₂:y₂:z₂ isused for forming the insulator 121 and the insulator 123, x₂/y₂ ispreferably less than x₁/y₁, and z₂/y₂ is preferably greater than orequal to 1/3 and less than or equal to 6 and further preferably greaterthan or equal to 1 and less than or equal to 6. Note that when z₂/y₂ isgreater than or equal to 1 and less than or equal to 6, a CAAC-OS filmis easily formed as the insulator 121 and the insulator 123. Typicalexamples of the atomic ratio of the metal elements of the target areIn:M:Zn=1:3:2, 1:3:4, 1:3:6, 1:3:8, 1:4:4, 1:4:5, 1:4:6, 1:4:7, 1:4:8,1:5:5, 1:5:6, 1:5:7, 1:5:8, 1:6:8, 1:6:4, 1:9:6, and the like.

In each of the insulator 121 and the insulator 123, the proportion ofeach atom in the above-described atomic ratio varies within a range of±40% as an error.

Alternatively, the insulator 123 can be metal oxide, such as aluminumoxide (AlO_(x)), gallium oxide (GaO_(x)), hafnium oxide (HfO_(x)),silicon oxide (SiO_(x)), germanium oxide (GeO_(x)), or zirconia oxide(ZrO_(x)); or the metal oxide may be provided over the insulator 123.

The atomic ratio is not limited to those described above, and may beappropriately set in accordance with needed semiconductorcharacteristics.

The insulator 121 and the insulator 123 may have the same composition.For example, the insulator 121 and the insulator 123 may be an In—Ga—Znoxide with an atomic ratio of the metal elements in a target used in asputtering method being In:Ga:Zn=1:3:2, 1:3:4, or 1:4:5.

Alternatively, the insulator 121 and the insulator 123 may havedifferent compositions. For example, the insulator 121 may be anIn—Ga—Zn oxide with an atomic ratio of the metal elements in a targetused in a sputtering method being In:Ga:Zn=1:3:4, and the insulator 123may be an In—Ga—Zn oxide with an atomic ratio of the metal elements in atarget used in a sputtering method being In:Ga:Zn=1:3:2.

The thickness of each of the insulator 121, the semiconductor layer 122,and the insulator 123 is preferably greater than or equal to 3 nm andless than or equal to 100 nm or greater than or equal to 3 nm and lessthan or equal to 50 nm.

The thickness of the semiconductor layer 122 may be larger than, equalto, or less than that of at least the insulator 121. If the thickness ofthe semiconductor layer 122 is larger than that of the insulator 121,the on-state current of the transistor can be increased. The thicknessof the insulator 121 may be determined as appropriate as long asformation of an interface state at the interface with the semiconductorlayer 122 can be suppressed. For example, the thickness of thesemiconductor layer 122 is larger than that of the insulator 121,preferably 2 or more times, further preferably 4 or more times, andstill further preferably 6 or more times, as large as that of theinsulator 121. In the case where there is no need to increase theon-state current of the transistor, the thickness of the insulator 121may be larger than or equal to that of the semiconductor layer 122. Ifthe insulating layer 110, 170, 173, or 175 contains excess oxygen, theoxygen is diffused by heat treatment and the amount of oxygen vacanciesin the semiconductor layer 122 can be reduced, which leads tostabilization of electrical characteristics of the semiconductor device.

The thickness of the insulator 123 may be determined as appropriate, ina manner similar to that of the insulator 121, as long as formation ofan interface state at the interface with the semiconductor layer 122 issuppressed. For example, the thickness of the insulator 123 may be setsmaller than or equal to that of the insulator 121. If the thickness ofthe insulator 123 is large, there is a concern that the electric fieldfrom the gate electrode layer 160 cannot reach the semiconductor layer122. To prevent oxygen contained in the insulator 123 from diffusing tothe source electrode layer 130 and the drain electrode layer 140 andoxidizing the source electrode layer 130 and the drain electrode layer140, it is preferable that the thickness of the insulator 123 be small.For example, the thickness of the insulator 123 is smaller than that ofthe semiconductor layer 122. Note that the thickness of the insulator123 is not limited to the above, and may be determined as appropriate inaccordance with the driving voltage of the transistor in considerationof the withstand voltage of the gate insulating layer 150.

In the case where the insulator 121, the semiconductor layer 122, andthe insulator 123 have different compositions from one another, theinterfaces thereof can be observed by scanning transmission electronmicroscopy (STEM) in some cases.

<Hydrogen Concentration>

Hydrogen contained in the insulator 121, the semiconductor layer 122,and the insulator 123 reacts with oxygen bonded to a metal atom to bewater, and in addition, an oxygen vacancy is formed in a lattice fromwhich oxygen is released (or a portion from which oxygen is released).An electron serving as a carrier is generated in some cases due to entryof hydrogen into the oxygen vacancy or due to bonding of part ofhydrogen to oxygen bonded to a metal atom. Thus, a transistor in whichan oxide containing hydrogen is used as a semiconductor is likely to benormally on.

Accordingly, it is preferable that hydrogen be reduced as much aspossible as well as the oxygen vacancies in the insulator 121, thesemiconductor layer 122, and the insulator 123 and at their interfaces.For example, the concentrations of hydrogen in the insulator 121, thesemiconductor layer 122, and the insulator 123 and at their interfaces,which are obtained by secondary ion mass spectrometry (SIMS), arepreferably higher than or equal to 1×10¹⁶ atoms/cm³ and lower than orequal to 2×10²⁰ atoms/cm³, further preferably higher than or equal to1×10¹⁶ atoms/cm³ and lower than or equal to 5×10¹⁹ atoms/cm³, stillfurther preferably higher than or equal to 1×10¹⁶ atoms/cm³ and lowerthan or equal to 1×10¹⁹ atoms/cm³, and yet still further preferablyhigher than or equal to 1×10¹⁶ atoms/cm³ and lower than or equal to5×10¹⁸ atoms/cm³. As a result, the transistor 10 can have positivethreshold voltage (normally-off characteristics).

<Concentrations of Carbon and Silicon>

When silicon and carbon, which are elements belonging to Group 14, arecontained in the insulator 121, the semiconductor layer 122, and theinsulator 123 and at their interfaces, oxygen vacancies are increasedand an n-type region is formed in the insulator 121, the semiconductorlayer 122, and the insulator 123. It is therefore preferable to reducethe concentrations of silicon and carbon in the insulator 121, thesemiconductor layer 122, and the insulator 123 and at their interfaces.For example, the concentrations of silicon and carbon in the insulator121, the semiconductor layer 122, and the insulator 123 and at theirinterfaces, which are obtained by SIMS, are preferably higher than orequal to 1×10¹⁶ atoms/cm³ and lower than or equal to 1×10¹⁹ atoms/cm³,further preferably higher than or equal to 1×10¹⁶ atoms/cm³ and lowerthan or equal to 5×10¹⁸ atoms/cm³, and still further preferably higherthan or equal to 1×10¹⁶ atoms/cm³ and lower than or equal to 2×10¹⁸atoms/cm³. As a result, the transistor 10 has positive threshold voltage(normally-off characteristics).

<Concentration of Alkali Metal and Alkaline Earth Metal>

Alkali metal and alkaline earth metal can generate carriers when bondedto an oxide, which can increase the off-state current of the transistor.It is thus preferable to reduce the concentrations of alkali metal andalkaline earth metal in the insulator 121, the semiconductor layer 122,and the insulator 123 and at their interfaces. For example, theconcentrations of alkali metal and alkaline earth metal in the insulator121, the semiconductor layer 122, and the insulator 123 and at theirinterfaces, which are obtained by SIMS, are preferably lower than orequal to 1×10¹⁸ atoms/cm³ and further preferably lower than or equal to2×10¹⁶ atoms/cm³. As a result, the transistor 10 has positive thresholdvoltage (normally-off characteristics).

<Concentration of Nitrogen>

When nitrogen is contained in the insulator 121, the semiconductor layer122, and the insulator 123 and at their interfaces, an electron servingas a carrier is generated and accordingly carrier density is increased,so that n-type regions are formed. Thus, when an oxide containsnitrogen, a transistor including the oxide is likely to be normally on.Thus, it is preferable that nitrogen be reduced as much as possible inthe insulator 121, the semiconductor layer 122, and the insulator 123and at their interfaces. For example, the concentrations of nitrogen inthe insulator 121, the semiconductor layer 122, and the insulator 123and at their interfaces, which are obtained by SIMS, are preferablyhigher than or equal to 1×10¹⁵ atoms/cm³ and lower than or equal to5×10¹⁹ atoms/cm³, further preferably higher than or equal to 1×10¹⁵atoms/cm³ and lower than or equal to 5×10¹⁸ atoms/cm³, still furtherpreferably higher than or equal to 1×10¹⁵ atoms/cm³ and lower than orequal to 1×10¹⁸ atoms/cm³, and yet still further preferably higher thanor equal to 1×10¹⁵ atoms/cm³ and lower than or equal to 5×10¹⁷atoms/cm³. As a result, the transistor 10 has positive threshold voltage(normally-off characteristics).

<Carrier Density>

The carrier densities of the insulator 121, the semiconductor layer 122,and the insulator 123 can be lowered by reduction in impurities in theinsulator 121, the semiconductor layer 122, and the insulator 123. Thecarrier densities of the insulator 121, the semiconductor layer 122, andthe insulator 123 is therefore less than or equal to 1×10¹⁵/cm³,preferably less than or equal to 1×10¹³/cm³, further preferably lessthan 8×10¹¹/cm³, still further preferably less than 1×10¹¹/cm³, and yetstill further preferably less than 1×10¹⁰/cm³, and is greater than orequal to 1×10⁻⁹/cm³.

When an oxide film having a low impurity concentration and a low densityof defect states is used as each of the insulator 121, the semiconductorlayer 122, and the insulator 123, a transistor including the insulatorsand the semiconductor layer can have more excellent electricalcharacteristics. Here, the state in which the impurity concentration islow and the density of defect states is low (the amount of oxygenvacancies is small) is described as “highly purified intrinsic” or“substantially highly purified intrinsic.” A highly purified intrinsicor substantially highly purified intrinsic oxide semiconductor has fewcarrier generation sources, and thus has a low carrier density in somecases. Thus, a transistor including the oxide film in which a channelregion is formed is likely to have positive threshold voltage(normally-off characteristics). A highly purified intrinsic orsubstantially highly purified intrinsic oxide film has a low density ofdefect states and accordingly has a low density of trap states in somecases. Further, a transistor including a highly purified intrinsic orsubstantially highly purified intrinsic oxide film has an extremely lowoff-state current; the off-state current can be less than or equal tothe measurement limit of a semiconductor parameter analyzer, i.e., lessthan or equal to 1×10⁻¹³ A, at a voltage between a source electrode anda drain electrode (drain voltage) of from 1 V to 10 V. Thus, thetransistor whose channel region is formed in the oxide film has a smallvariation in electrical characteristics and high reliability in somecases.

A transistor in which a highly purified oxide film is used for a channelformation region exhibits extremely low off-state current. For example,in the case where the voltage between the source and the drain is set toapproximately 0.1 V, 5 V, or 10 V, the off-state current standardized onthe channel width of the transistor can be as low as severalyoctoamperes per micrometer to several zeptoamperes per micrometer.

The insulator 121, the semiconductor layer 122, and the insulator 123may have a non-single crystal structure, for example. The non-singlecrystal structure includes a CAAC-OS which is described later, apolycrystalline structure, a microcrystalline structure, or an amorphousstructure, for example. Among the non-single crystal structure, theamorphous structure has the highest density of defect states, whereasthe CAAC-OS has the lowest density of defect states.

The insulator 121, the semiconductor layer 122, and the insulator 123may have a microcrystalline structure, for example. The insulator 121,the semiconductor layer 122, and the insulator 123 which each have themicrocrystalline structure include a microcrystal with a size greaterthan or equal to 1 nm and less than 10 nm, for example. Alternatively,the oxide films which have the microcrystalline structure have a mixedphase structure where crystal parts (each of which is greater than orequal to 1 nm and less than 10 nm) are distributed in an amorphousphase.

The insulator 121, the semiconductor layer 122, and the insulator 123may have an amorphous structure, for example. The insulator 121, thesemiconductor layer 122, and the insulator 123 which have the amorphousstructure each have disordered atomic arrangement and no crystallinecomponent, for example. Alternatively, the oxide films which have anamorphous structure have, for example, an absolutely amorphous structureand no crystal part.

Note that the insulator 121, the semiconductor layer 122, and theinsulator 123 may each be a mixed film including regions having two ormore of the following structures: a CAAC-OS, a microcrystallinestructure, and an amorphous structure. The mixed film, for example, hasa single-layer structure including a region having an amorphousstructure, a region having a microcrystalline structure, and a region ofa CAAC-OS. Alternatively, the mixed film may have a stacked-layerstructure including a region having an amorphous structure, a regionhaving a microcrystalline structure, and a region of a CAAC-OS, forexample.

Note that the insulator 121, the semiconductor layer 122, and theinsulator 123 may have a single-crystal structure, for example.

By providing an oxide film in which oxygen vacancies are less likely tobe generated than in the semiconductor layer 122, over and under and incontact with the semiconductor layer 122, oxygen vacancies in thesemiconductor layer 122 can be reduced. Further, since the semiconductorlayer 122 is in contact with the insulator 121 and the insulator 123containing one or more metal elements forming the semiconductor layer122, the density of interface states at the interfaces between theinsulator 121 and the semiconductor layer 122 and between thesemiconductor layer 122 and the insulator 123 is extremely low. Forexample, after oxygen is added to the insulating layer 110, the oxygenis transferred to the semiconductor layer 122 through the insulator 121by heat treatment; however, the oxygen is hardly trapped by theinterface states at this time, and the oxygen in the insulator 121 canbe efficiently transferred to the semiconductor layer 122. Accordingly,oxygen vacancies in the semiconductor layer 122 can be reduced. Sinceoxygen is added to the insulator 121, oxygen vacancies in the insulator121 can be reduced. In other words, the density of localized states ofat least the semiconductor layer 122 can be reduced.

In addition, when the semiconductor layer 122 is in contact with aninsulating film including a different constituent element (e.g., a gateinsulating layer including a silicon oxide film), an interface state issometimes formed, and the interface state forms a channel. At this time,a second transistor having a different threshold voltage appears, sothat an apparent threshold voltage of the transistor is varied. However,since the insulator 121 and the insulator 123 containing one or morekinds of metal elements forming the semiconductor layer 122 are incontact with the semiconductor layer 122, an interface state is noteasily formed at the interfaces between the insulator 121 and thesemiconductor layer 122 and between the insulator 123 and thesemiconductor layer 122.

The insulator 121 and the insulator 123 function as barrier films thatprevent constituent elements of the insulating layer 110 and the gateinsulating layer 150 from entering the semiconductor layer 122 andforming an impurity state.

For example, in the case of using a silicon-containing insulating filmas the insulating layer 110 or the gate insulating layer 150, silicon inthe gate insulating layer 150 or carbon which might be contained in theinsulating layer 110 or the gate insulating layer 150 enters theinsulator 121 or the insulator 123 to a depth of several nanometers fromthe interface in some cases. An impurity, such as silicon or carbon,entering the semiconductor layer 122 forms an impurity state. Theimpurity state serves as a donor to generate an electron; thus, ann-type semiconductor might be formed.

However, when each thickness of the insulator 121 and the insulator 123is greater than several nanometers, the impurity such as silicon orcarbon does not reach the semiconductor layer 122, so that the influenceof impurity states is reduced.

Thus, providing the insulator 121 and the insulator 123 makes itpossible to reduce variations in electrical characteristics of thetransistor, such as threshold voltage.

In the case where the gate insulating layer 150 and the semiconductorlayer 122 are in contact with each other and a channel is formed at theinterface thereof, interface scattering occurs at the interface and thefield-effect mobility of the transistor is decreased. However, since theinsulator 121 and the insulator 123 containing one or more kinds ofmetal elements forming the semiconductor layer 122 are provided incontact with the semiconductor layer 122, scattering of carriers doesnot easily occur at the interfaces between the semiconductor layer 122and each of the insulator 121 and the insulator 123, and thus thefield-effect mobility of the transistor can be increased.

In this embodiment, the amount of oxygen vacancies in the semiconductorlayer 122, and further the amount of oxygen vacancies in the insulator121 and the insulator 123 in contact with the semiconductor layer 122can be reduced; thus, the density of localized states of thesemiconductor layer 122 can be reduced. As a result, the transistor 10in this embodiment has small variations in threshold voltage and highreliability. Further, the transistor 10 of this embodiment has excellentelectrical characteristics.

An insulating film containing silicon is often used as a gate insulatinglayer of a transistor. For the above-described reason, it is preferablethat a region of the oxide layer, which serves as a channel, be not incontact with the gate insulating layer as in the transistor of oneembodiment of the present invention. In the case where a channel isformed at the interface between the gate insulating layer and the oxidelayer, scattering of carriers occurs at the interface, whereby thefield-effect mobility of the transistor is reduced in some cases. Alsofrom the view of the above, it is preferable that the region of theoxide layer, which serves as a channel, be separated from the gateinsulating layer.

Accordingly, with the semiconductor layer 120 having a stacked-layerstructure including the insulator 121, the semiconductor layer 122, andthe insulator 123, a channel can be formed in the semiconductor layer122; thus, the transistor can have high field-effect mobility and stableelectrical characteristics.

Note that the three semiconductor layers are not necessarily providedand can be a single layer, two layers, four layers, or five or morelayers. In the case of a single layer, a layer corresponding to thesemiconductor layer 122, which is described in this embodiment, can beused.

<Band Diagram>

Here, a band diagram is described. For easy understanding, the banddiagram is illustrated with the energy levels (Ec) at the conductionband minimum of the insulating layer 110, the insulator 121, thesemiconductor layer 122, the insulator 123, and the gate insulatinglayer 150.

As illustrated in FIGS. 3A and 3B, the energy at the conduction bandminimum changes continuously within the insulator 121, the semiconductorlayer 122, and the insulator 123. This can be understood also from thefact that the constituent elements are common among the insulator 121,the semiconductor layer 122, and the insulator 123 and oxygen is easilydiffused among them. Thus, the insulator 121, the semiconductor layer122, and the insulator 123 have a continuous physical property althoughthey are a stack of films having different compositions.

The oxide films, which contain the same main components and are stacked,are not simply stacked but formed to have continuous junction (here,particularly a U-shaped (U shape) well structure where the energy at theconduction band minimum is continuously changed between the films). Inother words, a stacked-layer structure is formed such that there existno impurities which form a defect level such as a trap center or arecombination center at each interface. If impurities are mixed betweenthe films in the stacked multilayer film, the continuity of the energyband is lost and carriers disappear by a trap or recombination at theinterface.

Although Ec of the insulator 121 and Ec of the insulator 123 are equalto each other in FIG. 3B, they may be different from each other.

As illustrated in FIG. 3B, the semiconductor layer 122 serves as a welland a channel of the transistor 10 is formed in the semiconductor layer122. Note that a channel having a U-shaped well structure in which theenergy at the conduction band minimum continuously changes like the oneformed in the semiconductor layer 122, can also be referred to as aburied channel.

Note that trap levels due to impurities or defects can be formed in thevicinity of the interface between an insulating film such as a siliconoxide film and the insulator 121 and the insulator 123. Thesemiconductor layer 122 can be distanced away from the trap levels owingto existence of the insulator 121 and the insulator 123. However, whenthe energy difference between Ec of the insulator 121 or the insulator123 and Ec of the semiconductor layer 122 is small, an electron in thesemiconductor layer 122 can go over the energy difference and reach thetrap level. When electrons to be negative charge are captured by thetrap levels, a negative fixed charge is generated at the interface withthe insulating film, whereby the threshold voltage of the transistor isshifted in the positive direction. In addition, a trap is not fixed andcharacteristics can be changed in a long-time preservation test of atransistor.

Thus, to reduce a change in the threshold voltage of the transistor, anenergy difference between the Ec of the semiconductor layer 122 and theEc of each of the insulator 121 and the insulator 123 is necessary. Theenergy difference is preferably greater than or equal to 0.1 eV andfurther preferably greater than or equal to 0.2 eV.

The insulator 121, the semiconductor layer 122, and the insulator 123preferably include a crystal part. In particular, when a crystal inwhich c-axes are aligned is used, the transistor can have stableelectrical characteristics.

In the band diagram illustrated in FIG. 3B, In—Ga oxide (e.g., with anatomic ratio of In:Ga=7:93) or gallium oxide may be provided between thesemiconductor layer 122 and the gate insulating layer 150 withoutproviding the insulator 123. Alternatively, In—Ga oxide or gallium oxidemay be provided between the insulator 123 and the gate insulating layer150.

As the semiconductor layer 122, an oxide having an electron affinityhigher than those of the insulators 121 and 123 is used. The oxide whichcan be used for the semiconductor layer 122 has, for example, anelectron affinity higher than that of each of the insulators 121 and 123by 0.07 eV or higher and 1.3 eV or lower, preferably 0.1 eV or higherand 0.7 eV or lower and further preferably 0.2 eV or higher and 0.4 eVor lower.

Since the transistor described in this embodiment includes the insulator121 and the insulator 123 that each include one or more kinds of metalelements included in the semiconductor layer 122, interface states canbe reduced at the interfaces between the insulator 121 and thesemiconductor layer 122 and between the insulator 123 and thesemiconductor layer 122. Thus, providing the insulator 121 and theinsulator 123 makes it possible to reduce variations or changes inelectrical characteristics of the transistor, such as threshold voltage.

<<Source Electrode Layer 130 and Drain Electrode Layer 140>>

The source electrode layer 130 and the drain electrode layer 140 arepreferably a conductive layer having a single-layer structure or astacked-layer structure and containing a material selected from copper(Cu), tungsten (W), molybdenum (Mo), gold (Au), aluminum (Al), manganese(Mn), titanium (Ti), tantalum (Ta), nickel (Ni), chromium (Cr), lead(Pb), tin (Sn), iron (Fe), cobalt (Co), ruthenium (Ru), platinum (Pt),iridium (Ir), strontium (Sr), and the like, an alloy of such a material,or a compound of oxygen, nitrogen, fluoride, silicon, or the likecontaining any of these materials as its main component. For example, inthe case of stacking layers, the lower conductive layer which is incontact with the semiconductor layer 122 contains a material which iseasily bonded to oxygen, and the upper conductive layer contains ahighly oxidation-resistant material. It is preferable to use ahigh-melting-point material, such as tungsten or molybdenum, which hasboth heat resistance and conductivity. In addition, the source electrodelayer 130 and the drain electrode layer 140 are preferably formed usinga low-resistance conductive material such as aluminum or copper. Thesource electrode layer 130 and the drain electrode layer 140 are furtherpreferably formed using a Cu—Mn alloy, in which case manganese oxideformed at the interface with an insulator containing oxygen has afunction of preventing Cu diffusion. In addition, tantalum nitride ispreferable because it has an effect of suppressing diffusion of hydrogenand oxygen (a barrier property) and shows high resistance to oxidation.

When the conductive material that is easily bonded to oxygen is incontact with an oxide semiconductor layer, a phenomenon occurs in whichoxygen in the oxide semiconductor layer is diffused to the conductivematerial that is easily bonded to oxygen. Oxygen vacancies are generatedin the vicinity of a region which is in the oxide semiconductor layerand is in contact with the source electrode layer or the drain electrodelayer. Hydrogen slightly contained in the film enters the oxygenvacancies, whereby the region is markedly changed to an n-type region.Accordingly, the n-type region can serve as a source or a drain of thetransistor.

For example, a stacked-layer structure using W and Pt for the lowerconductive layer and the upper conductive layer, respectively, cansuppress oxidation of the conductive layers caused by being in contactwith the insulating layer 175 while an oxide semiconductor in contactwith the conductive layers becomes n-type.

<<Gate Insulating Layer 150>>

The gate insulating layer 150 can contain oxygen (O), nitrogen (N),fluorine (F), aluminum (Al), magnesium (Mg), silicon (Si), gallium (Ga),germanium (Ge), yttrium (Y), zirconium (Zr), lanthanum (La), neodymium(Nd), hafnium (Hf), tantalum (Ta), titanium (Ti), or the like. Forexample, an insulating film containing one or more of aluminum oxide(AlO_(x)), magnesium oxide (MgO_(x)), silicon oxide (SiO_(x)), siliconoxynitride (SiO_(x)N_(y)), silicon nitride oxide (SiNx_(x)O_(y)),silicon nitride (SiN_(x)), gallium oxide (GaO_(x)), germanium oxide(GeO_(x)), yttrium oxide (YO_(x)), zirconium oxide (ZrO_(x)), lanthanumoxide (LaO_(x)), neodymium oxide (NdO_(x)), hafnium oxide (HfO_(x)), andtantalum oxide (TaO_(x)) can be used. The gate insulating layer 150 maybe a stack of any of the above materials. The gate insulating layer 150may contain lanthanum (La), nitrogen, zirconium (Zr), or the like as animpurity.

An example of a stacked-layer structure of the gate insulating layer 150is described. The gate insulating layer 150 includes, for example,oxygen, nitrogen, silicon, or hafnium. Specifically, the gate insulatinglayer 150 preferably includes hafnium oxide, and silicon oxide orsilicon oxynitride.

Hafnium oxide has higher dielectric constant than silicon oxide andsilicon oxynitride. Therefore, by using hafnium oxide, the thickness ofthe gate insulating layer 150 can be larger than that of silicon oxide;thus, leakage current due to tunnel current can be low. That is, it ispossible to provide a transistor with a low off-state current. Moreover,hafnium oxide with a crystalline structure has higher dielectricconstant than hafnium oxide with an amorphous structure. Therefore, itis preferable to use hafnium oxide with a crystalline structure in orderto provide a transistor with a low off-state current. Examples of thecrystalline structure include a monoclinic crystal structure and a cubiccrystal structure. Note that one embodiment of the present invention isnot limited to the above examples.

A surface over which the hafnium oxide with a crystalline structure isformed might have interface states due to defects. The interface stateserves as a trap center in some cases. Therefore, when hafnium oxide isprovided near a channel region of a transistor, the electricalcharacteristics of the transistor might deteriorate because of theinterface state. In order to reduce the adverse effect of the interfacestate, in some cases, it is preferable to separate the channel region ofthe transistor and the hafnium oxide from each other by providinganother film therebetween. The film has a buffer function. The filmhaving a buffer function may be included in the gate insulating layer150 or included in the oxide semiconductor film. That is, the filmhaving a buffer function can be formed using silicon oxide, siliconoxynitride, an oxide semiconductor, or the like. Note that the filmhaving a buffer function is formed using, for example, a semiconductoror an insulator having a larger energy gap than a semiconductor to bethe channel region. Alternatively, the film having a buffer function isformed using, for example, a semiconductor or an insulator having lowerelectron affinity than a semiconductor to be the channel region. Furtheralternatively, the film having a buffer function is formed using, forexample, a semiconductor or an insulator having higher ionization energythan a semiconductor to be the channel region.

In some cases, the threshold voltage of a transistor can be controlledby trapping an electric charge in an interface state (trap center) inhafnium oxide with the above-described crystalline structure in theformation surface where the hafnium oxide having the above-describedcrystalline structure is formed. In order to make the electric chargeexist stably, for example, an insulator having a larger energy gap thanhafnium oxide may be provided between the channel region and the hafniumoxide. Alternatively, a semiconductor or an insulator having lowerelectron affinity than hafnium oxide may be provided. The film having abuffer function may be formed using a semiconductor or an insulatorhaving higher ionization energy than hafnium oxide. With the use of suchan insulator, an electric charge trapped in the interface state is lesslikely to be released; accordingly, the electric charge can be held fora long period of time.

Examples of such an insulator include silicon oxide and siliconoxynitride. In order to make the interface state in the gate insulatinglayer 150 trap an electric charge, an electron may be transferred froman oxide semiconductor film toward the gate electrode layer 160. As aspecific example, the potential of the gate electrode layer 160 is kepthigher than the potential of the source electrode layer 130 or the drainelectrode layer 140 under high temperature conditions (e.g., atemperature higher than or equal to 125° C. and lower than or equal to450° C., typically higher than or equal to 150° C. and lower than orequal to 300° C.) for one second or longer, typically for one minute orlonger.

The threshold voltage of a transistor in which a predetermined amount ofelectrons are trapped in interface states in the gate insulating layer150 or the like shifts in the positive direction. The amount ofelectrons to be trapped (the amount of change in threshold voltage) canbe controlled by adjusting a voltage of the gate electrode layer 160 ortime in which the voltage is applied. Note that a location in which anelectric charge is trapped is not necessarily limited to the inside ofthe gate insulating layer 150 as long as an electric charge can betrapped therein. A stacked film having a similar structure may be usedas another insulating layer.

<<Gate Electrode Layer 160>>

For example, a conductive film of aluminum (Al), titanium (Ti), chromium(Cr), cobalt (Co), nickel (Ni), copper (Cu), yttrium (Y), zirconium(Zr), molybdenum (Mo), ruthenium (Ru), silver (Ag), tantalum (Ta),tungsten (W), or the like can be used for the gate electrode layer 160.The gate electrode layer 160 may have a stacked-layer structure.Alternatively, a conductive film containing nitrogen, such as a nitrideof the above material, may be used. In addition, tantalum nitride ispreferable because it has an effect of suppressing diffusion of hydrogenand oxygen (a barrier property) and shows high resistance to oxidation.

<<Insulating Layer 170>>

The insulating layer 170 can contain oxygen (O), nitrogen (N), fluorine(F), aluminum (Al), magnesium (Mg), silicon (Si), gallium (Ga),germanium (Ge), yttrium (Y), zirconium (Zr), lanthanum (La), neodymium(Nd), hafnium (Hf), tantalum (Ta), titanium (Ti), or the like. Forexample, an insulating film containing one or more of aluminum oxide(AlO_(x)), magnesium oxide (MgO_(x)), silicon oxide (SiO_(x)), siliconoxynitride (SiO_(x)N_(y)), silicon nitride oxide (SiN_(x)O_(y)), siliconnitride (SiN_(x)), gallium oxide (GaO_(x)), germanium oxide (GeO_(x)),yttrium oxide (YO_(x)), zirconium oxide (ZrO_(x)), lanthanum oxide(LaO_(x)), neodymium oxide (NdO_(x)), hafnium oxide (HfO_(x)), andtantalum oxide (TaO_(x)) can be used. The insulating layer 170 may be astack of any of the above materials.

Alternatively, an oxide containing In or Zn may be used for theinsulating layer 170. Typically, In—Ga oxide, In—Zn oxide, In—Mg oxide,Zn—Mg oxide, or In-M-Zn oxide (M is Al, Ti, Ga, Y, Zr, Sn, La, Ce, Mg,or Nd) can be given as the oxide.

An aluminum oxide film is preferably included in the insulating layer170. The aluminum oxide film can prevent transmission of both oxygen andimpurities, such as hydrogen and moisture. Accordingly, the aluminumoxide film is suitable for use as a protective film that has thefollowing prevention effects: during and after the manufacturing processof the transistor, entry of impurities, such as hydrogen and moisture,which cause variations in the electrical characteristics of thetransistor, into the insulator 121 and the semiconductor layer 122;release of oxygen, which is the main component, from the insulator 121and the semiconductor layer 122; and unnecessary release of oxygen fromthe insulating layer 110.

The insulating layer 170 is preferably a film having oxygen supplycapability. A mixed layer of the insulating layer 170 and the insulatinglayer 175 is formed and oxygen is added to the mixed layer or theinsulating layer 175 when an insulating film 170 a to be the insulatinglayer 170 is formed, the oxygen is diffused to an oxide semiconductor byheat treatment performed after that, and the oxygen can fill oxygenvacancies in the oxide semiconductor; therefore, the transistorcharacteristics (e.g., threshold voltage and reliability) can beimproved.

Alternatively, another insulating layer may be provided over or underthe insulating layer 170. For example, the insulating layer can beformed using an insulating film containing one or more of magnesiumoxide, silicon oxide, silicon oxynitride, silicon nitride oxide, siliconnitride, gallium oxide, germanium oxide, yttrium oxide, zirconium oxide,lanthanum oxide, neodymium oxide, hafnium oxide, and tantalum oxide. Theinsulating layer can contain oxygen (O), nitrogen (N), fluorine (F),aluminum (Al), magnesium (Mg), silicon (Si), gallium (Ga), germanium(Ge), yttrium (Y), zirconium (Zr), lanthanum (La), neodymium (Nd),hafnium (Hf), tantalum (Ta), titanium (Ti), or the like. For example, aninsulating film containing one or more of aluminum oxide (AlO_(x)),magnesium oxide (MgO_(x)), silicon oxide (SiO_(x)), silicon oxynitride(SiO_(x)N_(y)), silicon nitride oxide (SiN_(x)O_(y)), silicon nitride(SiN_(x)), gallium oxide (GaO_(x)), germanium oxide (GeO_(x)), yttriumoxide (YO_(x)), zirconium oxide (ZrO_(x)), lanthanum oxide (LaO_(x)),neodymium oxide (NdO_(x)), hafnium oxide (HfO_(x)), and tantalum oxide(TaO_(x)) can be used. The insulating layer may have a stacked-layerstructure. The insulating layer preferably contains oxygen more thanthat in the stoichiometric composition. Oxygen released from theinsulating layer can be diffused to the channel formation region in thesemiconductor layer 120 through the gate insulating layer 150, so thatoxygen vacancies formed in the channel formation region can be filledwith the oxygen. In this manner, stable electrical characteristics ofthe transistor can be achieved.

<<Insulating Layers 173 and 175>>

The insulating layers 173 and 175 can contain oxygen (O), nitrogen (N),fluorine (F), aluminum (Al), magnesium (Mg), silicon (Si), gallium (Ga),germanium (Ge), yttrium (Y), zirconium (Zr), lanthanum (La), neodymium(Nd), hafnium (Hf), tantalum (Ta), titanium (Ti), or the like. Forexample, an insulating film containing one or more of aluminum oxide(AlO_(x)), magnesium oxide (MgO_(x)), silicon oxide (SiO_(x)), siliconoxynitride (SiO_(x)N_(y)), silicon nitride oxide (SiN_(x)O_(y)), siliconnitride (SiN_(x)), gallium oxide (GaO_(x)), germanium oxide (GeO_(x)),yttrium oxide (YO_(x)), zirconium oxide (ZrO_(x)), lanthanum oxide(LaO_(x)), neodymium oxide (NdO_(x)), hafnium oxide (HfO_(x)), andtantalum oxide (TaO_(x)) can be used. The insulating layers 173 and 175each preferably contain oxygen more than that in the stoichiometriccomposition.

Alternatively, a low-dielectric constant material (a low-k material) maybe used for the insulating layers 173 and 175. For example, siliconoxide into which several percentage fluorine (F) is introduced (SiOF),silicon oxide into which several percentage carbon (C) is introduced(SiOC), fluorinesilicate glass (FSG), organosilicate glass (OSG),silsesquioxane hydride (HSQ), methylsilsesquioxane (MSQ), an organicpolymer, polyimide, a fluorine resin (e.g., polytetrafluoroethylene),amorphous carbon to which fluorine is added, or the like can be used.When the low-k material is used for the insulating layers 173 and 175,capacitance of the transistor 10 can be further reduced.

<Conductive Layer 165>

Note that the transistor 10 can include a conductive layer 165 under theinsulating layer 110 as illustrated in FIGS. 2A to 2D and the conductivelayer 165 can have a function as a bottom gate. The same potential asthe gate electrode layer 160 can be applied to the conductive layer asillustrated in FIG. 2B, or a potential different therefrom can beapplied to the conductive layer as illustrated in FIG. 2D. Theconductive layer 165 is preferably, for example, a conductive layerhaving a single-layer structure or a stacked-layer structure andcontaining a material selected from copper (Cu), tungsten (W),molybdenum (Mo), gold (Au), aluminum (Al), manganese (Mn), titanium(Ti), tantalum (Ta), nickel (Ni), chromium (Cr), lead (Pb), tin (Sn),iron (Fe), cobalt (Co), ruthenium (Ru), platinum (Pt), iridium (Ir),strontium (Sr), and the like, an alloy of such a material, or a compoundof oxygen, nitrogen, fluoride, silicon, or the like containing any ofthese materials as its main component. For example, a conductive layer166 can contain a highly oxidation-resistant material. It is preferableto use a high-melting-point material, such as tungsten or molybdenum,which has both heat resistance and conductivity for a conductive layer167. In addition, the conductive layer 167 is preferably formed using alow-resistance conductive material such as aluminum or copper.

<Manufacturing Method of Transistor>

Next, a manufacturing method of a semiconductor device of thisembodiment is described with reference to FIGS. 6A to 6C, FIGS. 7A to7C, FIGS. 8A to 8C, FIGS. 9A to 9C, FIGS. 10A to 10C, FIGS. 11A to 11C,FIGS. 12A to 12C, FIGS. 13A to 13C, and FIGS. 14A to 14C. Note that thesame parts as those in the above transistor structure are not describedhere. The direction of A1-A2 and that of A3-A4 in FIGS. 6A to 6C, FIGS.7A to 7C, FIGS. 8A to 8C, FIGS. 9A to 9C, FIGS. 10A to 10C, FIGS. 11A to11C, FIGS. 12A to 12C, FIGS. 13A to 13C, and FIGS. 14A to 14C can berespectively referred to as a channel length direction in FIGS. 1A and1B and a channel width direction in FIGS. 1A and 1C.

In this embodiment, the layers included in the transistor (i.e., theinsulating layer, the oxide semiconductor layer, the conductive layer,and the like) can be formed by any of a sputtering method, a chemicalvapor deposition (CVD) method, a vacuum evaporation method, and a pulsedlaser deposition (PLD) method. Alternatively, a coating method or aprinting method can be used. Although the sputtering method and aplasma-enhanced chemical vapor deposition (PECVD) method are typicalexamples of the film formation method, a thermal CVD method may be used.As the thermal CVD method, a metal organic chemical vapor deposition(MOCVD) method or an atomic layer deposition (ALD) method may be used,for example.

<Thermal CVD Method>

A thermal CVD method has an advantage that no defect due to plasmadamage is generated because it does not utilize plasma for forming afilm.

Deposition by a thermal CVD method may be performed in such a mannerthat a source gas and an oxidizer are supplied to the chamber at a time,the pressure in a chamber is set to an atmospheric pressure or a reducedpressure, and reaction is caused in the vicinity of the substrate orover the substrate.

The variety of films such as the metal film, the semiconductor film, andthe inorganic insulating film which have been disclosed in the aboveembodiments can be formed by a thermal CVD method such as a MOCVD methodor an ALD method. For example, in the case where an In—Ga—Zn—O film isformed, trimethylindium, trimethylgallium, and dimethylzinc can be used.Note that the chemical formula of trimethylindium is In(CH₃)₃. Thechemical formula of trimethylgallium is Ga(CH₃)₃. The chemical formulaof dimethylzinc is Zn(CH₃)₂. Without limitation to the abovecombination, triethylgallium (chemical formula: Ga(C₂H₅)₃) can be usedinstead of trimethylgallium, and diethylzinc (chemical formula:Zn(C₂H₅)₂) can be used instead of dimethylzinc.

<ALD Method>

In a conventional deposition apparatus utilizing a CVD method, one ormore kinds of source gases (precursors) for reaction are supplied to achamber at the same time at the time of deposition. In a depositionapparatus utilizing an ALD method, precursors for reaction aresequentially introduced into a chamber, and then the sequence of the gasintroduction is repeated. For example, two or more kinds of precursorsare sequentially supplied to the chamber by switching respectiveswitching valves (also referred to as high-speed valves). For example, afirst precursor is introduced, an inert gas (e.g., argon or nitrogen) orthe like is introduced after the introduction of the first precursor sothat the plural kinds of precursors are not mixed, and then a secondprecursor is introduced. Alternatively, the first precursor may beexhausted by vacuum evacuation instead of the introduction of the inertgas, and then the second precursor may be introduced.

FIGS. 4A to 4D illustrate a deposition process by an ALD method. Firstprecursors 601 are adsorbed onto a substrate surface (see FIG. 4A),whereby a first monolayer is formed (see FIG. 4B). At this time, metalatoms and the like included in the precursors can be bonded to hydroxylgroups that exist at the substrate surface. The metal atoms may bebonded to alkyl groups such as methyl groups or ethyl groups. The firstmonolayer reacts with second precursors 602 introduced after the firstprecursors 601 are evacuated (see FIG. 4C), whereby a second monolayeris stacked over the first monolayer. Thus, a thin film is formed (seeFIG. 4D). For example, in the case where an oxidizer is included in thesecond precursors, the oxidizer chemically reacts with metal atomsincluded in the first precursors or an alkyl group bonded to the metalatoms, whereby an oxide film can be formed. Moreover, with the use of agas containing hydrogen for the second precursors, a metal film can beformed by reduction reaction.

An ALD method is a deposition method based on a surface chemicalreaction, by which precursors are adsorbed onto a surface and adsorbingis stopped by a self-terminating mechanism, whereby a layer is formed.For example, precursors such as trimethylaluminum react with hydroxylgroups (OH groups) that exist at the surface. At this time, only asurface reaction due to heating occurs; therefore, the precursors comeinto contact with the surface and metal atoms or the like in theprecursors can be adsorbed onto the surface by thermal energy. Theprecursors have characteristics of, for example, having a high vaporpressure, being thermally stable before being deposited and notdissolving, and being chemically adsorbed onto a substrate at a highspeed. Since the precursors are introduced in a state of a gas, when thefirst precursors and the second precursors, which are alternatelyintroduced, have enough time to be diffused, a film can be formed withgood coverage even onto a region having unevenness with a high aspectratio.

In an ALD method, the sequence of the gas introduction is repeated aplurality of times until a desired thickness is obtained, whereby a thinfilm with excellent step coverage can be formed. The thickness of thethin film can be adjusted by the number of repetition times of thesequence of the gas introduction; therefore, an ALD method makes itpossible to accurately adjust a thickness. The deposition rate can beincreased and the impurity concentration in the film can be reduced byimproving the evacuation capability.

ALD methods include an ALD method using heating (thermal ALD method) andan ALD method using plasma (plasma ALD method). In the thermal ALDmethod, precursors react using thermal energy, and in the plasma ALDmethod, precursors react in a state of a radical.

By an ALD method, an extremely thin film can be formed with highaccuracy. In addition, the coverage of an uneven surface with the filmand the film density of the film are high.

<Plasma ALD>

Alternatively, when the plasma ALD method is employed, the film can beformed at a lower temperature than when the thermal ALD method isemployed. With the plasma ALD method, for example, the film can beformed without decreasing the deposition rate even at 100° C. or lower.Moreover, in the plasma ALD method, nitrogen radicals can be formed byplasma; thus, a nitride film as well as an oxide film can be formed.

In addition, oxidizability of an oxidizer can be enhanced by the plasmaALD method. Thus, precursors remaining in a plasma ALD film or organiccomponents released from precursors can be reduced. In addition, carbon,chlorine, hydrogen, and the like in the film can be reduced. Therefore,a film with low impurity concentration can be formed.

In the case of using the plasma ALD method, radical species aregenerated, and plasma can be generated from a place apart from thesubstrate like inductively coupled plasma (ICP) or the like, so thatplasma damage to the substrate or a film on which the protective film isformed can be suppressed.

As described above, with the plasma ALD method, the process temperaturecan be lowered and the coverage of the surface can be increased ascompared with other deposition methods, and the film can be depositedsuccessfully. Thus, entry of water and hydrogen from the outside can besuppressed, leading to an improvement of the reliability ofcharacteristics of the transistor.

<ALD Apparatus>

FIG. 5A illustrates an example of a deposition apparatus utilizing anALD method. The deposition apparatus utilizing an ALD method includes adeposition chamber (chamber 1701), source material supply portions 1711a and 1711 b, high-speed valves 1712 a and 1712 b which are flow ratecontrollers, source material introduction ports 1713 a and 1713 b, asource material exhaust port 1714, and an evacuation unit 1715. Thesource material introduction port 1713 a and the source materialintroduction port 1713 b provided in the chamber 1701 are connected tothe source material supply portion 1711 a and the source material supplyportion 1711 b, respectively, through supply tubes and valves. Thesource material exhaust port 1714 is connected to the evacuation unit1715 through an exhaust tube, a valve, and a pressure controller.

A substrate holder 1716 with a heater is provided in the chamber, and asubstrate 1700 over which a film is formed is provided over thesubstrate holder.

In the source material supply portions 1711 a and 1711 b, a source gasis formed from a solid source material or a liquid source material byusing a vaporizer, a heating unit, or the like. Alternatively, thesource material supply portions 1711 a and 1711 b may supply a sourcegas.

Although two source material supply portions 1711 a and 1711 b areprovided as an example, without limitation thereon, three or more sourcematerial supply portions may be provided. The high-speed valves 1712 aand 1712 b can be accurately controlled by time, and a source gas and aninert gas are supplied by the high-speed valves 1712 a and 1712 b. Thehigh-speed valves 1712 a and 1712 b are flow rate controllers for asource gas, and can also be referred to as flow rate controllers for aninert gas.

In the deposition apparatus illustrated in FIG. 5A, a thin film isformed over a surface of the substrate 1700 over which a film is formedin the following manner: the substrate 1700 over which a film is formedis transferred to put on the substrate holder 1716, the chamber 1701 issealed, the substrate 1700 over which a film is formed is heated to adesired temperature (e.g., higher than or equal to 100° C. or higherthan or equal to 150° C.) by heating the substrate holder 1716 with aheater; and supply of a source gas, evacuation with the evacuation unit1715, supply of an inert gas, and evacuation with the evacuation unit1715 are repeated.

In the deposition apparatus illustrated in FIG. 5A, an insulating layerformed using an oxide (including a composite oxide) containing one ormore elements selected from hafnium (Hf), aluminum (Al), tantalum (Ta),zirconium (Zr), and the like can be formed by selecting as appropriate asource material (e.g., a volatile organometallic compound) used for thesource material supply portions 1711 a and 1711 b. Specifically, it ispossible to use an insulating layer formed using hafnium oxide, aninsulating layer formed using aluminum oxide, an insulating layer formedusing hafnium silicate, an insulating layer formed using aluminumsilicate, or the like. Alternatively, a thin film, for example, a metallayer such as a tungsten layer or a titanium layer, or a nitride layersuch as a titanium nitride layer can be formed by selecting asappropriate a source material (e.g., a volatile organometallic compound)used for the source material supply portions 1711 a and 1711 b.

For example, in the case where a hafnium oxide layer is formed by adeposition apparatus using an ALD method, two kinds of gases, i.e.,ozone (O₃) as an oxidizer and a source gas which is obtained byvaporizing liquid containing a solvent and a hafnium precursor compound(hafnium alkoxide or hafnium amide such astetrakis(dimethylamide)hafnium (TDMAH)) are used. In this case, thefirst source gas supplied from the source material supply portion 1711 ais TDMAH, and the second source gas supplied from the source materialsupply portion 1711 b is ozone. Note that the chemical formula oftetrakis(dimethylamide)hafnium is Hf[N(CH₃)₂]₄. Examples of anothermaterial include tetrakis(ethylmethylamide)hafnium. Note that nitrogenhas a function of eliminating charge trap states. Therefore, when thesource gas contains nitrogen, a hafnium oxide film having low density ofcharge trap states can be formed.

For example, in the case where an aluminum oxide layer is formed by adeposition apparatus utilizing an ALD method, two kinds of gases, forexample, H₂O as an oxidizer and a source gas which is obtained byvaporizing liquid containing a solvent and an aluminum precursorcompound (e.g., trimethylaluminum (TMA)) are used. In this case, a firstsource gas supplied from the source material supply portion 1711 a isTMA, and a second source gas supplied from the source material supplyportion 1711 b is H₂O. Note that the chemical formula oftrimethylaluminum is Al(CH₃)₃. Examples of another material liquidinclude tris(dimethylamide)aluminum, triisobutylaluminum, and aluminumtris(2,2,6,6-tetramethyl-3,5-heptanedionate).

For example, in the case where a silicon oxide film is formed by adeposition apparatus using an ALD method, hexachlorodisilane is adsorbedon a surface where a film is to be formed, chlorine contained in theadsorbate is removed, and radicals of an oxidizing gas (e.g., O₂ ordinitrogen monoxide) are supplied to react with the adsorbate.

For example, in the case where a tungsten film is formed using adeposition apparatus employing ALD, a WF₆ gas and a B₂H₆ gas aresequentially introduced plural times to form an initial tungsten film,and then a WF₆ gas and an H₂ gas are introduced at a time, so that atungsten film is formed. Note that an SiH₄ gas may be used instead of aB₂H₆ gas.

For example, in the case where an oxide semiconductor film, for example,an In—Ga—Zn—O film is formed using a deposition apparatus employing ALD,an In(CH₃)₃ gas and an O₃ gas are sequentially introduced plural timesto form an InO₂ layer, a Ga(CH₃)₃ gas and an O₃ gas are introduced at atime to form a GaO layer, and then a Zn(CH₃)₂ gas and an O₃ gas areintroduced at a time to form a ZnO layer. Note that the order of theselayers is not limited to this example. A mixed compound layer such as anIn—Ga—O layer, an In—Zn—O layer, or a Ga—Zn—O layer may be formed byusing these gases. Note that although an H₂O gas which is obtained bybubbling with an inert gas such as Ar may be used instead of an O₃ gas,it is preferable to use an O₃ gas, which does not contain H. Instead ofan In(CH₃)₃ gas, an In(C₂H₅)₃ gas may be used. Instead of a Ga(CH₃)₃gas, a Ga(C₂H₅)₃ gas may be used. A Zn(CH₃)₂ gas may be used.

<<Multi-Chamber Manufacturing Apparatus>>

FIG. 5B Illustrates an Example of a Multi-Chamber ManufacturingApparatus including at least one deposition apparatus illustrated inFIG. 5A.

In the manufacturing apparatus illustrated in FIG. 5B, a stack of filmscan be successively formed without exposure to the air, and entry ofimpurities is prevented and throughput is improved.

The manufacturing apparatus illustrated in FIG. 5B includes at least aload chamber 1702, a transfer chamber 1720, a pretreatment chamber 1703,a chamber 1701 which is a deposition chamber, and an unload chamber1706. Note that in order to prevent attachment of moisture, the chambersof the manufacturing apparatus (including the load chamber, thetreatment chamber, the transfer chamber, the deposition chamber, theunload chamber, and the like) are preferably filled with an inert gas(such as a nitrogen gas) whose dew point is controlled, furtherpreferably maintain reduced pressure.

The chambers 1704 and 1705 may be deposition apparatuses utilizing anALD method like the chamber 1701, deposition apparatuses utilizing aplasma CVD method, deposition apparatuses utilizing a sputtering method,or deposition apparatuses utilizing a metal organic chemical vapordeposition (MOCVD) method.

For example, an example in which a stack of films is formed under acondition that the chamber 1704 is a deposition apparatus utilizing aplasma CVD method and the chamber 1705 is a deposition apparatusutilizing an MOCVD method is shown below.

Although FIG. 5B illustrates an example in which a top view of thetransfer chamber 1720 is a hexagon, a manufacturing apparatus in whichthe top surface shape is set to a polygon having more than six cornersand more chambers are connected depending on the number of layers of astack may be used. The top surface shape of the substrate is rectangularin FIG. 5B; however, there is no particular limitation on the topsurface shape of the substrate. Although FIG. 5B illustrates an exampleof the single wafer type, a batch-type deposition apparatus in which aplurality of substrates are formed at a time may be used.

<Formation of Insulating Layer 110>

First, the insulating layer 110 is formed over the substrate 100. Theinsulating layer 110 can be formed by a plasma CVD method, a thermal CVDmethod (an MOCVD method or an ALD method), a sputtering method, or thelike with the use of an oxide insulating film of aluminum oxide,magnesium oxide, silicon oxide, silicon oxynitride, gallium oxide,germanium oxide, yttrium oxide, zirconium oxide, lanthanum oxide,neodymium oxide, hafnium oxide, tantalum oxide, or the like; a nitrideinsulating film of silicon nitride, silicon nitride oxide, aluminumnitride, aluminum nitride oxide, or the like; or a mixed material of anyof these. Alternatively, these materials may be stacked, in which casean upper layer of the stacked layer which is in contact with a firstinsulator film to be the insulator 121 later is preferably formed usinga material containing excess oxygen that can serve as a supply source ofoxygen to the semiconductor layer 122.

For example, as the insulating layer 110, a 100-nm-thick siliconoxynitride film can be formed by a plasma CVD method.

Next, first heat treatment may be performed to release water, hydrogen,or the like contained in the insulating layer 110. As a result, theconcentration of water, hydrogen, or the like contained in theinsulating layer 110 can be reduced. The heat treatment can reduce theamount of water, hydrogen, or the like diffused to the first oxidesemiconductor film formed later.

<Formation of First Insulator Film and Semiconductor Film>

Then, a first insulator film to be the insulator 121 later and asemiconductor film to be the semiconductor layer 122 later are formedover the insulating layer 110. The first insulator film and thesemiconductor film can be formed by a sputtering method, an MOCVDmethod, a PLD method, or the like, and a sputtering method ispreferable. As a sputtering method, an RF sputtering method, a DCsputtering method, an AC sputtering method, or the like can be used. Inaddition, a facing-target-type sputtering method (also referred to as acounter-electrode-type sputtering method, a gas phase sputtering method,and a vapor deposition sputtering (VDSP) method) is used, whereby plasmadamage at the time of deposition can be reduced.

When the first insulator film is formed by a sputtering method, it ispreferable that each chamber of the sputtering apparatus be able to beevacuated to a high vacuum (about 5×10⁻⁷ Pa to 1×10⁻⁴ Pa) by anadsorption vacuum pump such as a cryopump and that the chamber be ableto heat a substrate over which a film is to be deposited to 100° C. orhigher, preferably 400° C. or higher, so that water and the like actingas impurities in the oxide semiconductor can be removed as much aspossible, for example. Alternatively, a combination of a turbo molecularpump and a cold trap is preferably used to prevent back-flow of a gascontaining a carbon component, moisture, or the like from an exhaustsystem into the chamber. Alternatively, a combination of a turbomolecular pump and a cryopump may be used as an exhaust system.

Not only high vacuum evacuation in a chamber but also high purity of asputtering gas is necessary to obtain a high-purity intrinsic oxidesemiconductor. When a highly purified gas having a dew point of −40° C.or lower, preferably −80° C. or lower and further preferably −100° C. orlower, is used as an oxygen gas or an argon gas used as a sputteringgas, moisture or the like can be prevented from entering an oxidesemiconductor as much as possible.

As a sputtering gas, a rare gas (typically argon), an oxygen gas, or amixed gas of a rare gas and an oxygen gas is used as appropriate. In thecase of using the mixed gas of a rare gas and an oxygen gas, theproportion of oxygen to a rare gas is preferably increased.

Note that, for example, in the case where the oxide semiconductor filmis formed by a sputtering method at a substrate temperature higher thanor equal to 150° C. and lower than or equal to 750° C., preferablyhigher than or equal to 150° C. and lower than or equal to 450° C. andfurther preferably higher than or equal to 200° C. and lower than orequal to 420° C., the oxide semiconductor film can be a CAAC-OS film.

The material of the first insulator film is selected so that the firstinsulator film can have a lower electron affinity than the semiconductorfilm.

The indium content in the semiconductor film is preferably higher thanthose in the first insulator film and a second insulator film. As asemiconductor, an s orbital of heavy metal mainly contributes to carriertransfer. When the proportion of In in the semiconductor is increased,overlap of s orbitals is likely to be increased. Therefore, an oxidehaving a composition in which the proportion of In is higher than thatof Ga has higher mobility than an oxide having a composition in whichthe proportion of In is equal to or lower than that of Ga. Thus, withthe use of an oxide having a high content of indium for thesemiconductor layer 122, a transistor having high field-effect mobilitycan be obtained.

When a sputtering method is used to form the first insulator film andthe semiconductor film, the first insulator film and the semiconductorfilm can be successively formed without exposing to the air with the useof a multi-chamber sputtering apparatus. In that case, entry ofunnecessary impurities and the like into the interface between the firstinsulator film and the semiconductor film can be prevented and thedensity of interface states can be reduced accordingly. Thus, theelectrical characteristics of a transistor can be stabilized,particularly in a reliability test.

If the semiconductor film is damaged, the semiconductor film, which is amain conduction path, can keep a distance from the damaged part owing tothe existence of the first insulator film. Thus, the electricalcharacteristics of a transistor can be stabilized, particularly in areliability test.

For example, as the first insulator film, a 20-nm-thick oxidesemiconductor film which is formed by a sputtering method using a targethaving an atomic ratio of In:Ga:Zn=1:3:4 can be used. In addition, asthe semiconductor film, a 15-nm-thick oxide semiconductor film which isformed by a sputtering method using a target having an atomic ratio ofIn:Ga:Zn=1:1:1 can be used.

It is preferable to perform second heat treatment after the firstinsulator film and the semiconductor film are formed. The amount ofoxygen vacancies in the semiconductor film can be reduced by performingthe second heat treatment.

The temperature of the second heat treatment is higher than or equal to250° C. and lower than the strain point of the substrate, preferablyhigher than or equal to 300° C. and lower than or equal to 650° C. andfurther preferably higher than or equal to 350° C. and lower than orequal to 550° C.

The second heat treatment is performed under an inert gas atmospherecontaining nitrogen or a rare gas such as helium, neon, argon, xenon, orkrypton. Further, after heat treatment performed in an inert gasatmosphere, heat treatment may be additionally performed in an oxygenatmosphere or a dry air atmosphere (air whose dew point is lower than orequal to −80° C., preferably lower than or equal to −100° C. and furtherpreferably lower than or equal to −120° C.). The treatment may beperformed under reduced pressure. Note that it is preferable thathydrogen, water, and the like be not contained in an inert gas and anoxygen gas, like the dry air, and the dew point is preferably lower thanor equal to −80° C., further preferably lower than or equal to −100° C.The treatment time is 3 minutes to 24 hours.

In the second heat treatment, instead of an electric furnace, any devicefor heating an object by heat conduction or heat radiation from aheating element, such as a resistance heating element, may be used. Forexample, an RTA (rapid thermal annealing) apparatus, such as a GRTA (gasrapid thermal annealing) apparatus or an LRTA (lamp rapid thermalannealing) apparatus, can be used. The LRTA apparatus is an apparatusfor heating an object to be processed by radiation of light (anelectromagnetic wave) emitted from a lamp, such as a halogen lamp, ametal halide lamp, a xenon arc lamp, a carbon arc lamp, a high pressuresodium lamp, or a high pressure mercury lamp. The GRTA apparatus is anapparatus for the second heat treatment using a high-temperature gas. Asthe high-temperature gas, an inert gas, such as nitrogen or a rare gaslike argon, is used.

Note that the second heat treatment may be performed after etching forforming the insulator 121 and the semiconductor layer 122 describedlater.

For example, after heat treatment is performed at 450° C. in a nitrogenatmosphere for one hour, heat treatment is performed at 450° C. in anoxygen atmosphere for one hour.

Through the above-described steps, oxygen vacancies and impurities suchas hydrogen and water in the semiconductor films can be reduced. Thesemiconductor films can have low density of localized states.

<Formation of First Conductive Film>

Next, a first conductive film to be the source electrode layer 130 andthe drain electrode layer 140 is formed over the semiconductor film. Thefirst conductive film can be formed by a sputtering method, a chemicalvapor deposition (CVD) method such as a metal organic chemical vapordeposition (MOCVD) method, a metal chemical deposition method, an atomiclayer deposition (ALD) method, or a plasma-enhanced chemical vapordeposition (PECVD) method, an evaporation method, a pulsed laserdeposition (PLD) method, or the like.

The first conductive film is preferably, for example, a conductive filmhaving a single-layer structure or a stacked-layer structure andcontaining a material selected from copper (Cu), tungsten (W),molybdenum (Mo), gold (Au), aluminum (Al), manganese (Mn), titanium(Ti), tantalum (Ta), nickel (Ni), chromium (Cr), lead (Pb), tin (Sn),iron (Fe), cobalt (Co), ruthenium (Ru), platinum (Pt), iridium (Ir),strontium (Sr), and the like, an alloy of such a material, or a compoundcontaining any of these materials as its main component. For example, inthe case of stacking layers, the lower conductive layer which is incontact with the semiconductor layer 122 contains a material which iseasily bonded to oxygen, and the upper conductive layer contains ahighly oxidation-resistant material. It is preferable to use ahigh-melting-point material, such as tungsten or molybdenum, which hasboth heat resistance and conductivity. In addition, the first conductivefilm is preferably formed using a low-resistance conductive materialsuch as aluminum or copper. The first conductive film is furtherpreferably formed using a Cu—Mn alloy, in which case a film containingmanganese oxide formed at the interface with an insulator containingoxygen has a function of preventing Cu diffusion.

For example, as the first conductive film, a tungsten film having athickness of 20 nm to 100 nm can be formed by a sputtering method.

A conductive layer 130 b formed by processing the first conductive filmin a subsequent step can have a function as a hard mask and a functionas a source electrode layer and a drain electrode layer in thesubsequent steps; thus, no additional step is needed. Thus, thesemiconductor manufacturing process can be shortened.

<Formation of Insulator 121 and Semiconductor Layer 122>

Then, a resist mask is formed through a lithography process. Part of thefirst conductive film is selectively etched using the resist mask, sothat the conductive layer 130 b is formed. The resist over theconductive layer 130 b is removed. The semiconductor film and the firstoxide semiconductor film are selectively etched using the conductivelayer 130 b as a hard mask, so that the island-shaped semiconductorlayer 122 and insulator 121 can be formed (see FIGS. 6A to 6C). A dryetching method can be used here. Note that the use of the conductivelayer 130 b as a hard mask for etching for the semiconductor film andthe first insulator film can reduce edge roughness of the etched oxidesemiconductor layer as compared to the case of using only a resist mask.

For example, the first oxide semiconductor film and the semiconductorfilm are selectively etched using a resist mask and a hard mask with amethane gas and an argon gas used as an etching gas, whereby theinsulator 121 and the semiconductor layer 122 can be formed. At thistime, the insulating layer 110 may be partly etched.

<Formation of Insulating Film>

Next, an insulating film 173 a to be the insulating layer 173 is formedover the insulating layer 110 and the conductive layer 130 b (see FIGS.7A to 7C). The insulating film 173 a can be formed by a plasma CVDmethod, a thermal CVD method (an MOCVD method or an ALD method), asputtering method, a spin coating method, or the like with the use of anoxide insulating film of aluminum oxide (AlO_(x)), magnesium oxide(MgO_(x)), silicon oxide (SiO_(x)), silicon oxynitride (SiO_(x)N_(y)),gallium oxide (GaO_(x)), germanium oxide (GeO_(x)), yttrium oxide(YO_(x)), zirconium oxide (ZrO_(x)), lanthanum oxide (LaO_(x)),neodymium oxide (NdO_(x)), hafnium oxide (HfO_(x)), tantalum oxide(TaO_(x)), or the like; a nitride insulating film of silicon nitride(SiN_(x)), silicon nitride oxide (SiN_(x)O_(y)), aluminum nitride(AlN_(x)), aluminum nitride oxide (AlN_(x)O_(y)), or the like; or amixed material of any of these. Alternatively, these materials may bestacked.

Alternatively, a low-dielectric constant material (a low-k material) maybe used for the insulating film 173 a. For example, silicon oxide intowhich several percentage fluorine (F) is introduced (SiOF), siliconoxide into which several percentage carbon (C) is introduced (SiOC),fluorinesilicate glass (FSG), organosilicate glass (OSG), silsesquioxanehydride (HSQ), methylsilsesquioxane (MSQ), an organic polymer,polyimide, a fluorine resin (e.g., polytetrafluoroethylene), amorphouscarbon to which fluorine is added, or the like can be used.

Note that the second heat treatment may be performed after the formationof the insulating film 173 a.

<Planarization of Second Insulating Film>

Then, planarization treatment is performed on the insulating film 173 auntil the conductive layer 130 b is exposed, so that the insulatinglayer 173 is formed (see FIGS. 8A to 8C). The planarization treatmentcan be performed by a chemical mechanical polishing (CMP) method or thelike. Accordingly, the thickness of an insulating film 175 a describedlater which is over the conductive layer 130 b in the substrate surfacecan be formed uniformly.

Note that the second heat treatment may be performed after theplanarization of the insulating film 173 a.

<Formation of Insulating Film>

Next, the third insulating film 175 a to be the insulating layer 175 isformed over the insulating layer 173 and the conductive layer 130 b (seeFIGS. 9A to 9C).

The insulating film 175 a can be formed using a material and a methodsimilar to those for the insulating film 173 a.

<Formation of Groove Portion>

Next, a resist mask 176 is formed over the insulating film 175 a by alithography process (see FIGS. 10A to 10C). Note that the lithographyprocess may be performed after an organic film is applied to theinsulating film 175 a or an organic film is applied to the resist. Theorganic film contains propylene glycolmonomethyl ether, ethyl lactate,or the like. The organic film has a function as an anti-reflection film(a bottom anti-reflective coating (BARC) film) during light exposure,and can improve adhesion to the resist, the resolution, and the like.

Note that in the case where a transistor having an extremely shortchannel length is formed, at least the conductive layer in a region todivide the conductive layer 130 b to be the source electrode layer 130and the drain electrode layer 140 is etched using a resist mask that isprocessed by a method suitable for micropatterning, such as electronbeam exposure, liquid immersion exposure, or extreme ultraviolet (EUV)exposure. Note that in the case of forming the resist mask by electronbeam exposure, a positive resist mask is used, so that an exposed regioncan be minimized and throughput can be improved. In the above manner, atransistor having a channel length of 100 nm or less, further, 30 nm orless can be formed. Alternatively, minute processing may be performed byan exposure technology which uses X-rays or the like.

With the resist mask, processing is partly performed on the insulatingfilm 175 a by a dry etching method. Accordingly, a groove portion 174 isformed at the same time as the formation of the insulating layer 175.

Then, the exposed conductive layer 130 b is partly etched to be divided,so that the source electrode layer 130 and the drain electrode layer 140can be formed (see FIGS. 11A to 11C).

After the source electrode layer 130 and the drain electrode layer 140are formed, cleaning treatment is preferably performed to remove anetching residue. The cleaning treatment can prevent a short circuitbetween the source electrode layer 130 and the drain electrode layer140. The cleaning treatment can be performed using an alkaline solutionsuch as a tetramethylammonium hydroxide (TMAH) solution, an acidicsolution such as diluted hydrofluoric acid, an oxalic acid solution, ora phosphoric acid solution. By the cleaning treatment, part of thesemiconductor layer 122 is etched and a depression is formed in thesemiconductor layer 122.

For example, the silicon oxynitride film formed as the insulating film173 a is planarized, a resist mask is formed over the silicon oxynitridefilm by a lithography method, an opening is formed in the siliconoxynitride film by a dry etching method using the resist mask and gascontaining carbon or fluorine, and dry etching is performed on theconductive layer 130 b using a chlorine-based gas or a fluorine-basedgas, whereby the source electrode layer 130 and the drain electrodelayer 140 can be formed.

<Formation of Second Insulator Film 123 a>

Next, a second insulator film 123 a to be the insulator 123 is formedover the semiconductor layer 122 and the insulating layer 175. Thesecond insulator film 123 a can be formed in a manner similar to that ofthe first insulator film. The materials can be selected such that theelectron affinity of the second insulator film 123 a is smaller thanthat of the semiconductor film.

For example, as the second insulator film 123 a, a 5-nm-thick oxidesemiconductor film which is formed by a sputtering method using a targethaving an atomic ratio of In:Ga:Zn=1:3:2 can be used.

<Formation of Insulating Film 150 a>

Next, an insulating film 150 a to be the gate insulating layer 150 isformed over the second insulator film 123 a. The insulating film 150 acan be formed using aluminum oxide (AlO_(x)), magnesium oxide (MgO_(x)),silicon oxide (SiO_(x)), silicon oxynitride (SiO_(x)N_(y)), siliconnitride oxide, silicon nitride, gallium oxide, germanium oxide, yttriumoxide, zirconium oxide, lanthanum oxide, neodymium oxide, hafnium oxide,tantalum oxide, or the like. The insulating film 150 a may be a stackcontaining any of these materials. The insulating film 150 a can beformed by a sputtering method, a CVD method such as a plasma CVD method,a MOCVD method, or an ALD method, an MBE method, or the like. Theinsulating film 150 a can be formed by a method similar to that of theinsulating layer 110 as appropriate.

For example, a 10-nm-thick silicon oxynitride film is formed by a plasmaCVD method as the insulating film 150 a.

<Formation of Conductive Film 160 a>

Next, a conductive film 160 a to be the gate electrode layer 160 isformed over the insulating film 150 a (see FIGS. 12A to 12C). Forexample, aluminum (Al), titanium (Ti), chromium (Cr), cobalt (Co),nickel (Ni), copper (Cu), yttrium (Y), zirconium (Zr), molybdenum (Mo),ruthenium (Ru), silver (Ag), gold (Au), platinum (Pt), tantalum (Ta),and tungsten (W), or an alloy material containing any of these as itsmain component can be used for the conductive film 160 a. The conductivefilm 160 a can be formed by a sputtering method, a CVD method such as aplasma CVD method, a MOCVD method, or an ALD method, an MBE method, anevaporation method, a plating method, or the like. The conductive film160 a may be formed using a conductive film containing nitrogen or astack including the conductive film and a conductive film containingnitrogen. Note that the conductive film 160 a may have either asingle-layer structure or a stacked-layer structure.

For example, a stack of a 10-nm-thick titanium nitride film formed by anALD method and a 150-nm-thick tungsten film formed by a metal CVD methodcan be used.

<Planarization Treatment>

Next, planarization treatment is performed. The planarization treatmentcan be performed by a CMP method, a dry etching method, or the like. Theplanarization treatment may be terminated at the time when theinsulating film 150 a is exposed, may be terminated at the time when thesecond insulator film 123 a is exposed, or may be terminated at the timewhen the insulating layer 175 is exposed. Accordingly, the gateelectrode layer 160, the gate insulating layer 150, and the insulator123 can be formed (see FIGS. 13A to 13C).

In the case where the second insulator film 123 a or the insulating film150 a is provided over the insulating layer 175 that has beenplanarized, another resist mask may be used for the processing. Theresist mask is formed over the second insulator film 123 a or theinsulating film 150 a by a lithography process. The mask has a largerarea than that of the top surface portion of the gate electrode layer160. The insulating film 150 a and the second insulator film 123 a areselectively etched using the mask, so that the gate insulating layer 150and the insulator 123 can be formed.

In the transistor 10, by providing the insulator 123 in which oxygenvacancies are unlikely to occur, release of oxygen from side surfaces ofthe insulator 123 in the channel width direction is suppressed, so thatgeneration of oxygen vacancies can be suppressed. As a result, atransistor which has improved electrical characteristics and highreliability can be provided.

<Formation of Insulating Layer 170>

Next, the insulating layer 170 is formed over the insulating layer 173,the insulator 123, the gate insulating layer 150, and the gate electrodelayer 160.

The insulating layer 170 can be formed by a plasma CVD method, a thermalCVD method (an MOCVD method or an ALD method), a sputtering method, orthe like with the use of an oxide insulating film of aluminum oxide(AlO_(x)), magnesium oxide (MgO_(x)), silicon oxide (SiO_(x)), siliconoxynitride (SiO_(x)N_(y)), gallium oxide (GaO_(x)), germanium oxide(GeO_(x)), yttrium oxide (YO_(x)), zirconium oxide (ZrO_(x)), lanthanumoxide (LaO_(x)), neodymium oxide (NdO_(x)), hafnium oxide (HfO_(x)),tantalum oxide (TaO_(x)), or the like; a nitride insulating film ofsilicon nitride (SiN_(x)), silicon nitride oxide (SiN_(x)O_(y)),aluminum nitride (AlN_(x)), aluminum nitride oxide (AlN_(x)O_(y)), orthe like; or a mixed material of any of these. Alternatively, thesematerials may be stacked. Alternatively, an oxide containing In or Znmay be used for the insulating layer 170. Typically, In—Ga oxide, In—Znoxide, In—Mg oxide, Zn—Mg oxide, or In-M-Zn oxide (M is Al, Ti, Ga, Y,Zr, Sn, La, Ce, Mg, or Nd) can be given as the oxide.

As the insulating layer 170, an aluminum oxide film is preferably formedby a sputtering method. Moreover, aluminum oxide is preferably used as asputtering target. An oxygen gas is preferably used as a gas used forforming the insulating layer 170.

A mixed layer 171 is formed at an interface with the insulating layer173 when the aluminum oxide film is formed.

For example, the oxygen gas used for forming the insulating layer 170exists in a variety of states such as an oxygen radical, an oxygen ion,or an oxygen atom during the formation by a sputtering method owing tothe influence of applied voltage, power, plasma, substrate temperature,or the like. At this time, oxygen (excess oxygen (exO)) 172 is added tothe insulating layer 173 or the mixed layer 171.

<Oxygen Addition>

The manufacturing method of the transistor 10 is not limited to theabove, and treatment for adding oxygen may be additionally performed.Oxygen may be added to the insulating layer 110, or the first oxidesemiconductor film or the second insulator film 123 a described above.As oxygen that is added, at least one kind selected from oxygenradicals, oxygen atoms, oxygen atomic ions, oxygen molecular ions, andthe like is used. As a method for adding oxygen, an ion doping method,an ion implantation method, a plasma immersion ion implantation method,or the like can be used.

In the case of using an ion implantation method as the method for addingoxygen, oxygen atomic ions or oxygen molecular ions can be used. The useof oxygen molecular ions can reduce damage to a film to which oxygen isadded. Oxygen molecular ions are broken down into oxygen atomic ions atthe surface of the film to which oxygen is added, and the oxygen atomicions are added. Since energy for breaking oxygen molecules down intooxygen atoms is used, the energy per oxygen atomic ion in the case ofadding oxygen molecular ions to the film to which oxygen is added islower than that in the case of adding oxygen atomic ions to the film towhich oxygen is added. Therefore, damage to the film to which oxygen isadded can be reduced.

In the case of injecting oxygen molecular ions, the energy per oxygenatomic ion is low as compared with the case of injecting oxygen atomicions. Thus, by using oxygen molecular ions for injection, theacceleration voltage can be increased and throughput can be increased.Moreover, by using oxygen molecular ions for injection, the same amountof oxygen atomic ions can be added at a dose half that necessary in thecase of using oxygen atomic ions. As a result, throughput of themanufacturing process can be increased.

In the case of adding oxygen to the film to which oxygen is added, it ispreferable that oxygen be added to the film to which oxygen is added sothat a peak of the concentration profile of oxygen atomic ions islocated in the film to which oxygen is added. In that case, theacceleration voltage for implantation can be lowered as compared to thecase where oxygen atomic ions are implanted, and damage to the film towhich oxygen is added can be reduced. In other words, defects in thefilm to which oxygen is added can be reduced, and variations inelectrical characteristics of the transistor can be suppressed. As aresult, damage to the film to which oxygen is added can be reduced,suppressing variation in the electrical characteristics of thetransistor.

Plasma treatment (plasma immersion ion implantation method) in which thefilm to which oxygen is added is exposed to plasma generated in anatmosphere containing oxygen may be performed to add oxygen to the filmto which oxygen is added. As the atmosphere containing oxygen, anatmosphere containing an oxidation gas such as oxygen, ozone, dinitrogenmonoxide, or nitrogen dioxide can be given. Note that it is preferablethat the film to which oxygen is added be exposed to plasma generated ina state where bias is applied on the substrate 100 side because theamount of oxygen added to the film to which oxygen is added can beincreased. As an example of an apparatus with which such plasmatreatment is performed, an ashing apparatus is given.

For example, oxygen molecular ions can be added to the first oxidesemiconductor film by an ion implantation method with a dose of1×10¹⁶/cm² at an acceleration voltage of 5 kV.

Through the above-described steps, which are combined with heattreatment in a later step, the amount of oxygen vacancies in thesemiconductor layer 122 can be reduced. Note that the film to whichoxygen is added has a low film density compared with the film to whichoxygen is not added.

Next, third heat treatment may be performed. The third heat treatmentcan be performed typically at a temperature higher than or equal to 150°C. and lower than the strain point of the substrate, preferably higherthan or equal to 250° C. and lower than or equal to 500° C. and furtherpreferably higher than or equal to 300° C. and lower than or equal to450° C. By the heat treatment, added oxygen 172 is diffused andtransferred to the semiconductor layer 122, and oxygen vacancies in thesemiconductor layer 122 can be filled with the oxygen (see FIGS. 14A to14C).

For example, the insulating layer 170 can be formed by a sputteringmethod with an aluminum oxide (AlO_(x)) target and 50 vol % of an oxygengas used as a sputtering gas. The thickness can be greater than or equalto 20 nm and less than or equal to 40 nm. The third heat treatment canbe performed at 400° C. in an oxygen atmosphere for one hour.

Through the above-described steps, the density of localized states ofthe semiconductor films is lowered, and thus a transistor with excellentelectrical characteristics can be manufactured. In addition, a highlyreliable transistor in which a variation in electrical characteristicswith time or a variation in electrical characteristics due to a stresstest is small can be manufactured.

Embodiment 2

In this embodiment, a method for manufacturing a transistor 11 and atransistor 12 each having a structure different from the structure ofthe transistor 10 described in Embodiment 1 will be described.

<Modification Example 1 of Transistor 10: Transistor 11>

The transistor 11 which is different in shape from the transistor 10illustrated in FIGS. 1A to 1C is described with reference to FIGS. 15Ato 15C.

FIGS. 15A to 15C are a top view and cross-sectional views of thetransistor 11. FIG. 15A is a top view of the transistor 11, FIG. 15B isa cross-sectional view taken along the dashed-dotted line A1-A2 in FIG.15A, and FIG. 15C is a cross-sectional view taken along thedashed-dotted line A3-4 in FIG. 15A.

The transistor 11 is different from the transistor 10 in that theinsulating layer 170 includes regions in contact with side surfaces ofthe insulator 121, the semiconductor layer 122, the source electrodelayer 130, the drain electrode layer 140, and the insulating layer 175.

Moreover, the manufacturing method of the transistor 11 is differentfrom that of the transistor 10 in that the insulator 121, thesemiconductor layer 122, the source electrode layer 130, the drainelectrode layer 140, and the insulating layer 175 are formed in the samestep and planarization treatment is not performed in each step, as willbe described later.

<Manufacturing Method of Transistor 11>

A manufacturing method of the transistor 11 is described with referenceto FIGS. 16A to 16C, FIGS. 17A to 17C, FIGS. 18A to 18C, FIGS. 19A to19C, FIGS. 20A to 20C, FIGS. 21A to 21D, FIGS. 22A to 22D, and FIGS. 23Ato 23C. Note that for steps similar to those of the transistor 10described in Embodiment 1, the description of the transistor is referredto.

As illustrated in FIGS. 16A and 16B, the insulating layer 110, a firstinsulator film 121 a to be the insulator 121, a semiconductor film 122 ato be the semiconductor layer 122, a conductive film 130 a to be thesource electrode layer 130 and the drain electrode layer 140, and theinsulating film 175 a to be the insulating layer 175 are formed, and aresist mask 176 is formed over the insulating film 175 a by alithography method.

Next, the insulating film 175 a and the conductive film 130 a are partlyetched with the resist mask 176, so that the groove portion 174, aninsulating layer 175 b, and the conductive layer 130 b are formed (seeFIGS. 17A to 17C).

The insulating layer 175 b, the conductive layer 130 b, thesemiconductor film 122 a, and the first insulator film 121 a are partlyetched with a resist mask, so that the insulator 121, the semiconductorlayer 122, the source electrode layer 130, the drain electrode layer140, and the insulating layer 175 are formed (see FIGS. 18A to 18C).

The second insulator film 123 a to be the insulator 123, the insulatingfilm 150 a to be the gate insulating layer 150, and the conductive film160 a to be the gate electrode layer 160 are sequentially formed (seeFIGS. 19A to 19C).

Then, the conductive film 160 a, the insulating film 150 a, and thesecond insulator film 123 a are partly etched with a resist mask formedby a lithography method, so that the insulator 123, the gate insulatinglayer 150, and the gate electrode layer 160 are formed (see FIGS. 20A to20C).

Note that the insulator 123, the gate insulating layer 150, and the gateelectrode layer 160 are not necessarily formed at a time. As illustratedin FIGS. 21A to 21D and FIGS. 22A to 22D, the insulator 123 and the gateelectrode layer 160 may be formed without etching the insulating film150 a. The insulator 123 may be formed so that its end portion does notoverlap with an end portion of the gate insulating layer 150 and an endportion of the gate electrode layer 160.

Next, the insulating layer 170 is formed and subjected to heattreatment, so that oxygen 172 is diffused to the semiconductor layer 122and oxygen vacancies in the semiconductor layer 122 can be reduced (seeFIGS. 23A to 23C).

Through the above-described steps, the transistor 11 can bemanufactured.

In the manufacturing method of the transistor 11, the insulating film175 a to be the insulating layer 175 can be formed before the conductivefilm 130 a to be the source electrode layer 130 and the drain electrodelayer 140 is processed, so that the thickness of the insulating film 175a can be uniform in the substrate surface and time for etching treatmentat the formation of the groove portion 174 can be stabilized.Accordingly, the transistor 11 can be manufactured stably and the shapeof the transistor can be stabilized. Thus, the transistorcharacteristics can be stabilized.

<Modification Example 2 of Transistor 10: Transistor 12>

The transistor 12 which is different in shape from the transistor 10illustrated in FIGS. 1A to 1C is described with reference to FIGS. 24Ato 24C.

FIGS. 24A to 24C are a top view and cross-sectional views of thetransistor 12. FIG. 24A is a top view of the transistor 12, FIG. 24B isa cross-sectional view taken along the dashed-dotted line A1-A2 in FIG.24A, and FIG. 24C is a cross-sectional view taken along thedashed-dotted line A3-4 in FIG. 24A.

The transistor 12 is different from the transistor 10 in including aninsulating layer 177.

The manufacturing method is different from that of the transistor 10 inthat the first insulator film 121 a to be the insulator 121, thesemiconductor film 122 a to be the semiconductor layer 122, theconductive film 130 a to be the source electrode layer 130 and the drainelectrode layer 140, the insulating film 175 a to be the insulatinglayer 175, and an insulating film 177 a to be the insulating layer 177are formed before being processed, as will be described later.

<Manufacturing Method of Transistor 12>

The manufacturing method of the transistor 12 is described withreference to FIGS. 25A to 25C, FIGS. 26A to 26C, FIGS. 27A to 27C, FIGS.28A to 28C, FIGS. 29A to 29C, FIGS. 30A to 30C, FIGS. 31A to 31C, andFIGS. 32A to 32C. Note that for steps similar to those of the transistor10 described in Embodiment 1, the description of the transistor isreferred to.

Over the insulating layer 110, the first insulator film 121 a to be theinsulator 121, the semiconductor film 122 a to be the semiconductorlayer 122, the conductive film 130 a to be the source electrode layer130 and the drain electrode layer 140, the insulating film 175 a to bethe insulating layer 175, and the insulating film 177 a to be theinsulating layer 177 are sequentially formed before being processed (seeFIGS. 25A to 25C).

The insulating layer 177 can be formed using a material and a methodsimilar to those for the insulating layer 170.

Next, the insulating film 177 a, the insulating film 175 a, theconductive film 130 a, the semiconductor film 122 a, and the firstinsulator film 121 a are partly etched with a resist mask, so that theinsulator 121, the semiconductor layer 122, the conductive layer 130 b,the insulating layer 175 b, and an insulating layer 171 b are formed(see FIGS. 26A to 26C). At this time, the insulating layer 110 may bepartly etched.

Next, the insulating film 173 a to be the insulating layer 173 is formed(see FIGS. 27A to 27C). The insulating film 173 a can be formed by aplasma CVD method, a thermal CVD method (an MOCVD method or an ALDmethod), a sputtering method, a spin coating method, or the like.

Then, planarization treatment by CMP is performed on the insulating film173 a until the insulating layer 171 b is exposed, so that theinsulating layer 173 is formed (see FIGS. 28A to 28C). Note that theinsulating layer 171 b preferably has a function as a stopper, i.e. hasa lower polishing rate than the insulating layer 173 under the same CMPprocessing conditions.

Then, a resist mask is formed through a lithography process over theinsulating layer 173 and the insulating layer 171 b. Part of theconductive layer 130 b is selectively etched using the insulating layer171 b as a hard mask until the semiconductor layer 122 is exposed.Accordingly, the source electrode layer 130, the drain electrode layer140, and the insulating layer 175 are formed (see FIGS. 29A to 29C).

The second insulator film 123 a, the insulating film 150 a, and theconductive film 160 a are sequentially formed (see FIGS. 30A to 30C).

Then, planarization treatment is performed on the conductive film 160 a,the insulating film 150 a, and the second insulator film 123 a, so thatthe insulator 123, the gate insulating layer 150, and the gate electrodelayer 160 are formed (see FIGS. 31A to 31C).

Next, the insulating layer 170 is formed and subjected to heattreatment, so that oxygen 172 is diffused to the semiconductor layer 122and oxygen vacancies in the semiconductor layer 122 can be reduced (seeFIGS. 32A to 32C).

Through the above-described steps, the transistor 12 can bemanufactured.

In the manufacturing method of the transistor 12, the insulating film175 a to be the insulating layer 175 can be formed before the conductivefilm 130 a to be the source electrode layer 130 and the drain electrodelayer 140 is processed, so that the thickness of the insulating film 175a can be uniform in the substrate surface and time for etching treatmentat the formation of the groove portion 174 can be stabilized.Accordingly, the transistor 12 can be manufactured stably and the shapeof the transistor can be stabilized. Thus, the transistorcharacteristics (e.g., a threshold voltage or reliability) can bestabilized.

This embodiment can be combined as appropriate with any of the otherembodiments in this specification.

Embodiment 3

<Oxide Semiconductor Structure>

In this embodiment, the structure of an oxide semiconductor will bedescribed.

An oxide semiconductor is classified into a single crystal oxidesemiconductor and a non-single-crystal oxide semiconductor. Examples ofa non-single-crystal oxide semiconductor include a c-axis alignedcrystalline oxide semiconductor (CAAC-OS), a polycrystalline oxidesemiconductor, a nanocrystalline oxide semiconductor (nc-OS), anamorphous-like oxide semiconductor (a-like OS), and an amorphous oxidesemiconductor

From another perspective, an oxide semiconductor is classified into anamorphous oxide semiconductor and a crystalline oxide semiconductor.Examples of a crystalline oxide semiconductor include a single crystaloxide semiconductor, a CAAC-OS, a polycrystalline oxide semiconductor,and an nc-OS.

It is known that an amorphous structure is generally defined as beingmetastable and unfixed, and being isotropic and having no non-uniformstructure. In other words, an amorphous structure has a flexible bondangle and a short-range order but does not have a long-range order.

This means that an inherently stable oxide semiconductor cannot beregarded as a completely amorphous oxide semiconductor. Moreover, anoxide semiconductor that is not isotropic (e.g., an oxide semiconductorthat has a periodic structure in a microscopic region) cannot beregarded as a completely amorphous oxide semiconductor. Note that ana-like OS has a periodic structure in a microscopic region, but at thesame time has a void and has an unstable structure. For this reason, ana-like OS has physical properties similar to those of an amorphous oxidesemiconductor.

<CAAC-OS>

First, a CAAC-OS is described.

A CAAC-OS is one of oxide semiconductors having a plurality of c-axisaligned crystal parts (also referred to as pellets).

In a combined analysis image (also referred to as a high-resolution TEMimage) of a bright-field image and a diffraction pattern of a CAAC-OS,which is obtained using a transmission electron microscope (TEM), aplurality of pellets can be observed. However, in the high-resolutionTEM image, a boundary between pellets, that is, a grain boundary is notclearly observed. Thus, in the CAAC-OS, a reduction in electron mobilitydue to the grain boundary is less likely to occur.

The CAAC-OS observed with a TEM is described below. FIG. 33A shows ahigh-resolution TEM image of a cross section of the CAAC-OS which isobserved from a direction substantially parallel to the sample surface.The high-resolution TEM image is obtained with a spherical aberrationcorrector function. The high-resolution TEM image obtained with aspherical aberration corrector function is particularly referred to as aCs-corrected high-resolution TEM image. The Cs-corrected high-resolutionTEM image can be obtained with, for example, an atomic resolutionanalytical electron microscope JEM-ARM200F manufactured by JEOL Ltd.

FIG. 33B is an enlarged Cs-corrected high-resolution TEM image of aregion (1) in FIG. 33A. FIG. 33B shows that metal atoms are arranged ina layered manner in a pellet. Each metal atom layer has a configurationreflecting unevenness of a surface over which the CAAC-OS is formed(hereinafter, the surface is referred to as a formation surface) or atop surface of the CAAC-OS, and is arranged parallel to the formationsurface or the top surface of the CAAC-OS.

As shown in FIG. 33B, the CAAC-OS film has a characteristic atomicarrangement. The characteristic atomic arrangement is denoted by anauxiliary line in FIG. 33C. FIGS. 33B and 33C prove that the size of apellet is greater than or equal to 1 nm or greater than or equal to 3nm, and the size of a space caused by tilt of the pellets isapproximately 0.8 nm. Therefore, the pellet can also be referred to as ananocrystal (nc). Furthermore, the CAAC-OS can also be referred to as anoxide semiconductor including c-axis aligned nanocrystals (CANC).

Here, according to the Cs-corrected high-resolution TEM images, theschematic arrangement of pellets 5100 of a CAAC-OS layer over asubstrate 5120 is illustrated by such a structure in which bricks orblocks are stacked (see FIG. 33D). The part in which the pellets aretilted as observed in FIG. 33C corresponds to a region 5161 illustratedin FIG. 33D.

FIG. 34A shows a Cs-corrected high-resolution TEM image of a plane ofthe CAAC-OS observed from a direction substantially perpendicular to thesample surface. FIGS. 34B, 34C, and 34D are enlarged Cs-correctedhigh-resolution TEM images of regions (1), (2), and (3) in FIG. 34A,respectively. FIGS. 34B, 34C, and 34D indicate that metal atoms arearranged in a triangular, quadrangular, or hexagonal configuration in apellet. However, there is no regularity of arrangement of metal atomsbetween different pellets.

Next, a CAAC-OS analyzed by X-ray diffraction (XRD) is described. Forexample, when the structure of a CAAC-OS including an InGaZnO₄ crystalis analyzed by an out-of-plane method, a peak appears at a diffractionangle (2θ) of around 31° as shown in FIG. 35A. This peak is derived fromthe (009) plane of the InGaZnO₄ crystal, which indicates that crystalsin the CAAC-OS have c-axis alignment, and that the c-axes are aligned ina direction substantially perpendicular to the formation surface or thetop surface of the CAAC-OS.

Note that in structural analysis of the CAAC-OS by an out-of-planemethod, another peak may appear when 2θ is around 36°, in addition tothe peak at 2θ of around 31°. The peak of 2θ at around 36° indicatesthat a crystal having no c-axis alignment is included in part of theCAAC-OS. It is preferable that in the CAAC-OS analyzed by anout-of-plane method, a peak appear when 28 is around 31° and that a peaknot appear when 2θ is around 36°.

On the other hand, in structural analysis of the CAAC-OS by an in-planemethod in which an X-ray is incident on a sample in a directionsubstantially perpendicular to the c-axis, a peak appears when 2θ isaround 56°. This peak is derived from the (110) plane of the InGaZnO₄crystal. In the case of the CAAC-OS, when analysis (ϕ scan) is performedwith 2θ fixed at around 56° and with the sample rotated using a normalvector of the sample surface as an axis (ϕ axis), as shown in FIG. 35B,a peak is not clearly observed. In contrast, in the case of a singlecrystal oxide semiconductor of InGaZnO₄, when ϕ scan is performed with2θ fixed at around 56°, as shown in FIG. 35C, six peaks which arederived from crystal planes equivalent to the (110) plane are observed.Accordingly, the structural analysis using XRD shows that the directionsof a-axes and b-axes are irregularly oriented in the CAAC-OS.

Next, a CAAC-OS analyzed by electron diffraction is described. Forexample, when an electron beam with a probe diameter of 300 nm isincident on a CAAC-OS including an InGaZnO₄ crystal in a directionparallel to the sample surface, a diffraction pattern (also referred toas a selected-area transmission electron diffraction pattern) shown inFIG. 36A can be obtained. In this diffraction pattern, spots derivedfrom the (009) plane of an InGaZnO₄ crystal are included. Thus, theelectron diffraction also indicates that pellets included in the CAAC-OShave c-axis alignment and that the c-axes are aligned in a directionsubstantially perpendicular to the formation surface or the top surfaceof the CAAC-OS. Meanwhile, FIG. 36B shows a diffraction pattern obtainedin such a manner that an electron beam with a probe diameter of 300 nmis incident on the same sample in a direction perpendicular to thesample surface. As shown in FIG. 36B, a ring-like diffraction pattern isobserved. Thus, the electron diffraction also indicates that the a-axesand b-axes of the pellets included in the CAAC-OS do not have regularalignment. The first ring in FIG. 36B is considered to be derived fromthe (010) plane, the (100) plane, and the like of the InGaZnO₄ crystal.Furthermore, it is supposed that the second ring in FIG. 36B is derivedfrom the (110) plane and the like.

As described above, the CAAC-OS is an oxide semiconductor with highcrystallinity. Entry of impurities, formation of defects, or the likemight decrease the crystallinity of an oxide semiconductor. This meansthat the CAAC-OS has small amounts of impurities and defects (e.g.,oxygen vacancies).

Note that the impurity means an element other than the main componentsof the oxide semiconductor, such as hydrogen, carbon, silicon, or atransition metal element. For example, an element (specifically, siliconor the like) having higher strength of bonding to oxygen than a metalelement included in an oxide semiconductor extracts oxygen from theoxide semiconductor, which results in disorder of the atomic arrangementand reduced crystallinity of the oxide semiconductor. A heavy metal suchas iron or nickel, argon, carbon dioxide, or the like has a large atomicradius (or molecular radius), and thus disturbs the atomic arrangementof the oxide semiconductor and decreases crystallinity.

The characteristics of an oxide semiconductor having impurities ordefects might be changed by light, heat, or the like. Impuritiesincluded in the oxide semiconductor might serve as carrier traps orcarrier generation sources, for example. Furthermore, oxygen vacancy inthe oxide semiconductor serves as a carrier trap or serves as a carriergeneration source when hydrogen is captured therein.

The CAAC-OS having small amounts of impurities and oxygen vacancies isan oxide semiconductor film with low carrier density (specifically,lower than 8×10¹¹/cm³, preferably lower than 1×10¹¹/cm³, and furtherpreferably lower than 1×10¹⁰/cm³, and is higher than or equal to1×10⁻⁹/cm³). Such an oxide semiconductor is referred to as a highlypurified intrinsic or substantially highly purified intrinsic oxidesemiconductor. A CAAC-OS has a low impurity concentration and a lowdensity of defect states. Thus, the CAAC-OS can be referred to as anoxide semiconductor having stable characteristics.

<nc-OS>

Next, an nc-OS is described.

An nc-OS has a region in which a crystal part is observed and a regionin which a crystal part is not clearly observed in a high-resolution TEMimage. In most cases, the size of a crystal part included in the nc-OSis greater than or equal to 1 nm and less than or equal to 10 nm, orgreater than or equal to 1 nm and less than or equal to 3 nm. Note thatan oxide semiconductor including a crystal part whose size is greaterthan 10 nm and less than or equal to 100 nm is sometimes referred to asa microcrystalline oxide semiconductor. In a high-resolution TEM imageof the nc-OS, for example, a grain boundary is not clearly observed insome cases. Note that there is a possibility that the origin of thenanocrystal is the same as that of a pellet in a CAAC-OS. Therefore, acrystal part of the nc-OS may be referred to as a pellet in thefollowing description.

In the nc-OS, a microscopic region (e.g., a region with a size greaterthan or equal to 1 nm and less than or equal to 10 nm, in particular, aregion with a size greater than or equal to 1 nm and less than or equalto 3 nm) has a periodic atomic arrangement. There is no regularity ofcrystal orientation between different pellets in the nc-OS. Thus, theorientation of the whole film is not observed. Accordingly, the nc-OScannot be distinguished from an a-like OS and an amorphous oxidesemiconductor, depending on an analysis method. For example, when thenc-OS is analyzed by an out-of-plane method using an X-ray beam having adiameter larger than the size of a pellet, a peak which shows a crystalplane does not appear. Furthermore, a diffraction pattern like a halopattern is observed when the nc-OS is subjected to electron diffractionusing an electron beam with a probe diameter (e.g., 50 nm or larger)that is larger than the size of a pellet. Meanwhile, spots appear in ananobeam electron diffraction pattern of the nc-OS when an electron beamhaving a probe diameter close to or smaller than the size of a pellet isapplied. Moreover, in a nanobeam electron diffraction pattern of thenc-OS, regions with high luminance in a circular (ring) pattern areshown in some cases. Also in a nanobeam electron diffraction pattern ofthe nc-OS layer, a plurality of spots is shown in a ring-like region insome cases.

Since there is no regularity of crystal orientation between the pellets(nanocrystals) as mentioned above, the nc-OS can also be referred to asan oxide semiconductor including random aligned nanocrystals (RANC) oran oxide semiconductor including non-aligned nanocrystals (NANC).

The nc-OS is an oxide semiconductor that has high regularity as comparedto an amorphous oxide semiconductor. Therefore, the nc-OS is likely tohave a lower density of defect states than an a-like OS and an amorphousoxide semiconductor. Note that there is no regularity of crystalorientation between different pellets in the nc-OS. Therefore, the nc-OShas a higher density of defect states than the CAAC-OS.

<A-Like OS>

An a-like OS is an oxide semiconductor having a structure between thenc-OS and the amorphous oxide semiconductor.

In a high-resolution TEM image of the a-like OS, a void may be observed.Furthermore, in the high-resolution TEM image, there are a region wherea crystal part is clearly observed and a region where a crystal part isnot observed.

The a-like OS has an unstable structure because it contains a void. Toverify that an a-like OS has an unstable structure as compared with aCAAC-OS and an nc-OS, a change in structure caused by electronirradiation is described below.

An a-like OS (sample A), an nc-OS (sample B), and a CAAC-OS (sample C)are prepared as samples subjected to electron irradiation. Each of thesamples is an In—Ga—Zn oxide.

First, a high-resolution cross-sectional TEM image of each sample isobtained. The high-resolution cross-sectional TEM images show that allthe samples have crystal parts.

Note that which part is regarded as a crystal part is determined asfollows. It is known that a unit cell of the InGaZnO₄ crystal has astructure in which nine layers including three In—O layers and sixGa—Zn—O layers are stacked in the c-axis direction. The distance betweenthe adjacent layers is equivalent to the lattice spacing on the (009)plane (also referred to as d value). The value is calculated to be 0.29nm from crystal structural analysis. Accordingly, a portion where thelattice spacing between lattice fringes is greater than or equal to 0.28nm and less than or equal to 0.30 nm is regarded as a crystal part ofInGaZnO₄. Each of lattice fringes corresponds to the a-b plane of theInGaZnO₄ crystal.

FIG. 37 shows change in the average size of crystal parts (at 22 pointsto 45 points) in each sample. Note that the crystal part sizecorresponds to the length of a lattice fringe. FIG. 37 indicates thatthe crystal part size in the a-like OS increases with an increase in thecumulative electron dose. Specifically, as shown by (1) in FIG. 37, acrystal part of approximately 1.2 nm at the start of TEM observation(the crystal part is also referred to as an initial nucleus) grows to asize of approximately 2.6 nm at a cumulative electron dose of 4.2×10⁸e⁻/nm². In contrast, the crystal part size in the nc-OS and the CAAC-OSshows little change from the start of electron irradiation to acumulative electron dose of 4.2×10⁸ e⁻/nm². Specifically, as shown by(2) and (3) in FIG. 37, the average crystal sizes in an nc-OS and aCAAC-OS are approximately 1.4 nm and approximately 2.1 nm, respectively,regardless of the cumulative electron dose.

In this manner, growth of the crystal part in the a-like OS is inducedby electron irradiation. In contrast, in the nc-OS and the CAAC-OS,growth of the crystal part is hardly induced by electron irradiation.Therefore, the a-like OS has an unstable structure as compared with thenc-OS and the CAAC-OS.

The a-like OS has a lower density than the nc-OS and the CAAC-OS becauseit contains a void. Specifically, the density of the a-like OS is higherthan or equal to 78.6% and lower than 92.3% of the density of the singlecrystal oxide semiconductor having the same composition. The density ofeach of the nc-OS and the CAAC-OS is higher than or equal to 92.3% andlower than 100% of the density of the single crystal oxide semiconductorhaving the same composition. Note that it is difficult to deposit anoxide semiconductor having a density of lower than 78% of the density ofthe single crystal oxide semiconductor.

For example, in the case of an oxide semiconductor having an atomicratio of In:Ga:Zn=1:1:1, the density of single crystal InGaZnO₄ with arhombohedral crystal structure is 6.357 g/cm³. Accordingly, in the caseof the oxide semiconductor having an atomic ratio of In:Ga:Zn=1:1:1, thedensity of the a-like OS is higher than or equal to 5.0 g/cm³ and lowerthan 5.9 g/cm³. For example, in the case of the oxide semiconductorhaving an atomic ratio of In:Ga:Zn=1:1:1, the density of each of thenc-OS and the CAAC-OS is higher than or equal to 5.9 g/cm³ and lowerthan 6.3 g/cm³.

Note that there is a possibility that an oxide semiconductor having acertain composition cannot exist in a single crystal structure. In thatcase, single crystal oxide semiconductors with different compositionsare combined at an adequate ratio, which makes it possible to calculatedensity equivalent to that of a single crystal oxide semiconductor withthe desired composition. The density of a single crystal oxidesemiconductor having the desired composition can be calculated using aweighted average according to the combination ratio of the singlecrystal oxide semiconductors with different compositions. Note that itis preferable to use as few kinds of single crystal oxide semiconductorsas possible to calculate the density.

As described above, oxide semiconductors have various structures andvarious properties. Note that an oxide semiconductor may be a stackedlayer including two or more of an amorphous oxide semiconductor, ana-like OS, an nc-OS, and a CAAC-OS, for example.

Embodiment 4

In this embodiment, an example of a circuit including the transistor ofone embodiment of the present invention will be described with referenceto drawings

<Cross-Sectional Structure>

FIG. 38A is a cross-sectional view of a semiconductor device of oneembodiment of the present invention. In FIG. 38A, X1-X2 direction andY1-Y2 direction represents a channel length direction and a channelwidth direction, respectively. The semiconductor device illustrated inFIG. 38A includes a transistor 2200 containing a first semiconductormaterial in a lower portion and a transistor 2100 containing a secondsemiconductor material in an upper portion. In FIG. 38A, an example isdescribed in which the transistor described in the above embodiment asan example is used as the transistor 2100 containing the secondsemiconductor material. A cross-sectional view of the transistors in achannel length direction is on the left side of a dashed-dotted line,and a cross-sectional view of the transistors in a channel widthdirection is on the right side of the dashed-dotted line.

Here, the first semiconductor material and the second semiconductormaterial are preferably materials having different band gaps. Forexample, the first semiconductor material can be a semiconductormaterial other than an oxide semiconductor (examples of such asemiconductor material include silicon (including strained silicon),germanium, silicon germanium, silicon carbide, gallium arsenide,aluminum gallium arsenide, indium phosphide, gallium nitride, and anorganic semiconductor), and the second semiconductor material can be anoxide semiconductor. A transistor including a material other than anoxide semiconductor, such as single crystal silicon, can operate at highspeed easily. In contrast, a transistor including an oxide semiconductorand described in the above embodiment as an example has excellentsubthreshold characteristics and a minute structure. Furthermore, thetransistor can operate at a high speed because of its high switchingspeed and has low leakage current because of its low off-state current.

The transistor 2200 may be either an n-channel transistor or a p-channeltransistor, and an appropriate transistor may be used in accordance witha circuit. Furthermore, the specific structure of the semiconductordevice, such as the material or the structure used for the semiconductordevice, is not necessarily limited to those described here except forthe use of the transistor of one embodiment of the present inventionwhich uses an oxide semiconductor.

FIG. 38A illustrates a structure in which the transistor 2100 isprovided over the transistor 2200 with an insulator 2201 and aninsulator 2207 provided therebetween. A plurality of wirings 2202 areprovided between the transistor 2200 and the transistor 2100.Furthermore, wirings and electrodes provided over and under theinsulators are electrically connected to each other through a pluralityof plugs 2203 embedded in the insulators. An insulator 2204 covering thetransistor 2100 and a wiring 2205 over the insulator 2204 are provided.

The stack of the two kinds of transistors reduces the area occupied bythe circuit, allowing a plurality of circuits to be highly integrated.

Here, in the case where a silicon-based semiconductor material is usedfor the transistor 2200 provided in a lower portion, hydrogen in aninsulator provided in the vicinity of the semiconductor film of thetransistor 2200 terminates dangling bonds of silicon; accordingly, thereliability of the transistor 2200 can be improved. Meanwhile, in thecase where an oxide semiconductor is used for the transistor 2100provided in an upper portion, hydrogen in an insulator provided in thevicinity of the semiconductor film of the transistor 2100 becomes afactor of generating carriers in the oxide semiconductor; thus, thereliability of the transistor 2100 might be decreased. Therefore, in thecase where the transistor 2100 using an oxide semiconductor is providedover the transistor 2200 using a silicon-based semiconductor material,it is particularly effective that the insulator 2207 having a functionof preventing diffusion of hydrogen is provided between the transistors2100 and 2200. The insulator 2207 makes hydrogen remain in the lowerportion, thereby improving the reliability of the transistor 2200. Inaddition, since the insulator 2207 suppresses diffusion of hydrogen fromthe lower portion to the upper portion, the reliability of thetransistor 2100 can also be improved.

The insulator 2207 can be, for example, formed using aluminum oxide,aluminum oxynitride, gallium oxide, gallium oxynitride, yttrium oxide,yttrium oxynitride, hafnium oxide, hafnium oxynitride, oryttria-stabilized zirconia (YSZ).

Furthermore, a blocking film having a function of preventing diffusionof hydrogen is preferably formed over the transistor 2100 to cover thetransistor 2100 including an oxide semiconductor film. For the blockingfilm, a material that is similar to that of the insulator 2207 can beused, and in particular, an aluminum oxide film is preferably used. Thealuminum oxide film has a high shielding (blocking) effect of preventingtransmission of both oxygen and impurities such as hydrogen andmoisture. Thus, by using the aluminum oxide film as the blocking filmcovering the transistor 2100, release of oxygen from the oxidesemiconductor film included in the transistor 2100 can be prevented andentry of water and hydrogen into the oxide semiconductor film can beprevented. Note that as the blocking film, the insulator 2204 having astacked-layer structure may be used, or the blocking film may beprovided under the insulator 2204.

Note that the transistor 2200 can be a transistor of various typeswithout being limited to a planar type transistor. For example, thetransistor 2200 can be a fin-type transistor, a tri-gate transistor, orthe like. An example of a cross-sectional view in this case is shown inFIG. 38D. An insulator 2212 is provided over a semiconductor substrate2211. The semiconductor substrate 2211 includes a projecting portionwith a thin tip (also referred to a fin). Note that an insulator may beprovided over the projecting portion. The insulator functions as a maskfor preventing the semiconductor substrate 2211 from being etched whenthe projecting portion is formed. The projecting portion does notnecessarily have the thin tip; a projecting portion with a cuboid-likeprojecting portion and a projecting portion with a thick tip arepermitted, for example. A gate insulator 2214 is provided over theprojecting portion of the semiconductor substrate 2211, and a gateelectrode 2213 is provided over the gate insulator 2214. Source anddrain regions 2215 are formed in the semiconductor substrate 2211. Notethat here is shown an example in which the semiconductor substrate 2211includes the projecting portion; however, a semiconductor device of oneembodiment of the present invention is not limited thereto. For example,a semiconductor region having a projecting portion may be formed byprocessing an SOI substrate.

<Circuit Configuration Example>

In the above structure, electrodes of the transistor 2100 and thetransistor 2200 can be connected as appropriate; thus, a variety ofcircuits can be formed. Examples of circuit configurations which can beachieved by using a semiconductor device of one embodiment of thepresent invention are shown below.

<CMOS Inverter Circuit>

A circuit diagram in FIG. 38B illustrates a configuration of a CMOSinverter in which the p-channel transistor 2200 and the n-channeltransistor 2100 are connected to each other in series and in which gatesof them are connected to each other.

<CMOS Analog Switch>

A circuit diagram in FIG. 38C illustrates a configuration in whichsources of the transistors 2100 and 2200 are connected to each other anddrains of the transistors 2100 and 2200 are connected to each other.With such a configuration, the transistors can function as a CMOS analogswitch.

<Memory Device Example>

An example of a semiconductor device (memory device) which includes thetransistor of one embodiment of the present invention, which can retainstored data even when not powered, and which has an unlimited number ofwrite cycles is illustrated in FIGS. 39A to 39C.

The semiconductor device illustrated in FIG. 39A includes a transistor3200 using a first semiconductor material, a transistor 3300 using asecond semiconductor material, and a capacitor 3400. Note that any ofthe transistors in Embodiment 1 and 2 can be used as the transistor3300.

FIG. 39B is a cross-sectional view of the semiconductor deviceillustrated in FIG. 39A. The semiconductor device in the cross-sectionalview has a structure in which the transistor 3300 is provided with aback gate; however, a structure without a back gate may be employed.

The transistor 3300 is a transistor in which a channel is formed in asemiconductor layer including an oxide semiconductor. Since theoff-state current of the transistor 3300 is low, stored data can beretained for a long period. In other words, power consumption can besufficiently reduced because a semiconductor memory device in whichrefresh operation is unnecessary or the frequency of refresh operationis extremely low can be provided.

In FIG. 39A, a first wiring 3001 is electrically connected to a sourceelectrode of the transistor 3200. A second wiring 3002 is electricallyconnected to a drain electrode of the transistor 3200. A third wiring3003 is electrically connected to one of a source electrode and a drainelectrode of the transistor 3300. A fourth wiring 3004 is electricallyconnected to a gate electrode of the transistor 3300. A gate electrodeof the transistor 3200 is electrically connected to the other of thesource electrode and the drain electrode of the transistor 3300 and oneelectrode of the capacitor 3400. The fifth wiring 3005 is electricallyconnected to the other electrode of the capacitor 3400.

The semiconductor device in FIG. 39A has a feature that the potential ofthe gate electrode of the transistor 3200 can be retained, and thusenables writing, retaining, and reading of data as follows.

Writing and retaining of data are described. First, the potential of thefourth wiring 3004 is set to a potential at which the transistor 3300 isturned on, so that the transistor 3300 is turned on. Accordingly, thepotential of the third wiring 3003 is supplied to the gate electrode ofthe transistor 3200 and the capacitor 3400. That is, a predeterminedcharge is supplied to the gate electrode of the transistor 3200(writing). Here, one of two kinds of charges providing differentpotential levels (hereinafter referred to as a low-level charge and ahigh-level charge) is supplied. After that, the potential of the fourthwiring 3004 is set to a potential at which the transistor 3300 is turnedoff, so that the transistor 3300 is turned off. Thus, the chargesupplied to the gate electrode of the transistor 3200 is held(retaining).

Since the off-state current of the transistor 3300 is extremely low, thecharge of the gate of the transistor 3200 is retained for a long time.

Next, reading of data is described. An appropriate potential (a readingpotential) is supplied to the fifth wiring 3005 while a predeterminedpotential (a constant potential) is supplied to the first wiring 3001,whereby the potential of the second wiring 3002 varies depending on theamount of charge retained in the gate electrode of the transistor 3200.This is because in the case of using an n-channel transistor as thetransistor 3200, an apparent threshold voltage V_(th) _(_) _(H) at thetime when the high-level charge is given to the gate electrode of thetransistor 3200 is lower than an apparent threshold voltage V_(th) _(_)_(L) at the time when the low-level charge is given to the gateelectrode of the transistor 3200. Here, an apparent threshold voltagerefers to the potential of the fifth wiring 3005 which is needed to turnon the transistor 3200. Thus, the potential of the fifth wiring 3005 isset to a potential V₀ which is between V_(th) _(_) _(H) and V_(th) _(_)_(L), whereby charge supplied to the gate electrode of the transistor3200 can be determined. For example, in the case where the high-levelcharge is supplied to the gate electrode of the transistor 3200 inwriting and the potential of the fifth wiring 3005 is V₀ (>V_(th) _(_)_(H)), the transistor 3200 is turned on. In the case where the low-levelcharge is supplied to the gate electrode of the transistor 3200 inwriting, even when the potential of the fifth wiring 3005 is V₀ (<V_(th)_(_) _(L)), the transistor 3200 remains off. Thus, the data retained inthe gate electrode of the transistor 3200 can be read by determining thepotential of the second wiring 3002.

Note that in the case where memory cells are arrayed to be used, it isnecessary that only data of a desired memory cell be able to be read.For example, the fifth wiring 3005 of memory cells from which data isnot read may be supplied with a potential at which the transistor 3200is turned off regardless of the state of the gate electrode, that is, apotential lower than V_(th) _(_) _(H), whereby only data of a desiredmemory cell can be read. Alternatively, the fifth wiring 3005 of thememory cells from which data is not read may be supplied with apotential at which the transistor 3200 is turned on regardless of thestate of the gate electrode, that is, a potential higher than V_(th)_(_) _(L), whereby only data of a desired memory cell can be read.

The semiconductor device illustrated in FIG. 39C is different from thesemiconductor device illustrated in FIG. 39A in that the transistor 3200is not provided. Also in this case, writing and retaining operation ofdata can be performed in a manner similar to those of the semiconductordevice illustrated in FIG. 39A.

Next, reading of data is described. When the transistor 3300 is turnedon, the third wiring 3003 which is in a floating state and the capacitor3400 are electrically connected to each other, and the charge isredistributed between the third wiring 3003 and the capacitor 3400. As aresult, the potential of the third wiring 3003 is changed. The amount ofchange in the potential of the third wiring 3003 varies depending on thepotential of the one electrode of the capacitor 3400 (or the chargeaccumulated in the capacitor 3400).

For example, the potential of the third wiring 3003 after the chargeredistribution is (C_(B)×V_(B0)+C×V)/(C_(B)+C), where V is the potentialof the one electrode of the capacitor 3400, C is the capacitance of thecapacitor 3400, C_(B) is the capacitance component of the third wiring3003, and V_(B0) is the potential of the third wiring 3003 before thecharge redistribution. Thus, it can be found that, assuming that thememory cell is in either of two states in which the potential of the oneelectrode of the capacitor 3400 is V₁ and V₀ (V₁>V₀), the potential ofthe third wiring 3003 in the case of retaining the potential V₁(=(C_(B)×V_(B0)+C×V₁)/(C_(B)+C)) is higher than the potential of thethird wiring 3003 in the case of retaining the potential V₀(=(C_(B)×V_(B0)+C×V₀)/(C_(B)+C)).

Then, by comparing the potential of the third wiring 3003 with apredetermined potential, data can be read.

In this case, a transistor including the first semiconductor materialmay be used for a driver circuit for driving a memory cell, and atransistor including the second semiconductor material may be stackedover the driver circuit as the transistor 3300.

When including a transistor in which a channel formation region isformed using an oxide semiconductor and which has an extremely lowoff-state current, the semiconductor device described in this embodimentcan retain stored data for an extremely long time. In other words,refresh operation becomes unnecessary or the frequency of the refreshoperation can be extremely low, which leads to a sufficient reduction inpower consumption. Moreover, stored data can be retained for a long timeeven when power is not supplied (note that a potential is preferablyfixed).

Further, in the semiconductor device described in this embodiment, highvoltage is not needed for writing data and there is no problem ofdeterioration of elements. Unlike in a conventional nonvolatile memory,for example, it is not necessary to inject and extract electrons intoand from a floating gate; thus, a problem such as deterioration of agate insulating layer hardly occurs. That is, the semiconductor deviceof the disclosed invention does not have a limit on the number of timesdata can be rewritten, which is a problem of a conventional nonvolatilememory, and the reliability thereof is drastically improved.Furthermore, data is written depending on the state of the transistor(on or off), whereby high-speed operation can be easily achieved.

Note that in this specification and the like, it might be possible forthose skilled in the art to constitute one embodiment of the inventioneven when portions to which all the terminals of an active element(e.g., a transistor or a diode), a passive element (e.g., a capacitor ora resistor), or the like are connected are not specified. In otherwords, one embodiment of the invention can be clear even when connectionportions are not specified. Further, in the case where a connectionportion is disclosed in this specification and the like, it can bedetermined that one embodiment of the invention in which a connectionportion is not specified is disclosed in this specification and thelike, in some cases. In particular, in the case where the number ofportions to which the terminal is connected might be plural, it is notnecessary to specify the portions to which the terminal is connected.Therefore, it might be possible to constitute one embodiment of theinvention by specifying only portions to which some of terminals of anactive element (e.g., a transistor or a diode), a passive element (e.g.,a capacitor or a resistor), or the like are connected.

Note that in this specification and the like, it might be possible forthose skilled in the art to specify the invention when at least theconnection portion of a circuit is specified. Alternatively, it might bepossible for those skilled in the art to specify the invention when atleast a function of a circuit is specified. In other words, when afunction of a circuit is specified, one embodiment of the invention canbe clear. Furthermore, it can be determined that one embodiment of theinvention whose function is specified is disclosed in this specificationand the like. Therefore, when a connection portion of a circuit isspecified, the circuit is disclosed as one embodiment of the inventioneven when a function is not specified, and one embodiment of theinvention can be constituted. Alternatively, when a function of acircuit is specified, the circuit is disclosed as one embodiment of theinvention even when a connection portion is not specified, and oneembodiment of the invention can be constituted.

Note that in this specification and the like, in a diagram or a textdescribed in one embodiment, it is possible to take out part of thediagram or the text and constitute an embodiment of the invention. Thus,in the case where a diagram or a text related to a certain portion isdescribed, the context taken out from part of the diagram or the text isalso disclosed as one embodiment of the invention, and one embodiment ofthe invention can be constituted. Therefore, for example, in a diagramor text in which one or more active elements (e.g., transistors ordiodes), wirings, passive elements (e.g., capacitors or resistors),conductive layers, insulating layers, semiconductor layers, organicmaterials, inorganic materials, components, devices, operating methods,manufacturing methods, or the like are described, part of the diagram orthe text is taken out, and one embodiment of the invention can beconstituted. For example, from a circuit diagram in which N circuitelements (e.g., transistors or capacitors; N is an integer) areprovided, it is possible to constitute one embodiment of the inventionby taking out M circuit elements (e.g., transistors or capacitors; M isan integer, where M<N). As another example, it is possible to constituteone embodiment of the invention by taking out M layers (M is an integer,where M<N) from a cross-sectional view in which N layers (N is aninteger) are provided. As another example, it is possible to constituteone embodiment of the invention by taking out M elements (M is aninteger, where M<N) from a flow chart in which N elements (N is aninteger) are provided.

<Imaging Device>

An imaging device of one embodiment of the present invention isdescribed below.

FIG. 40A is a plan view illustrating an example of an imaging device 200of one embodiment of the present invention. The imaging device 200includes a pixel portion 210 and peripheral circuits for driving thepixel portion 210 (a peripheral circuit 260, a peripheral circuit 270, aperipheral circuit 280, and a peripheral circuit 290). The pixel portion210 includes a plurality of pixels 211 arranged in a matrix with p rowsand q columns (p and q are each a natural number greater than or equalto 2). The peripheral circuit 260, the peripheral circuit 270, theperipheral circuit 280, and the peripheral circuit 290 are eachconnected to a plurality of pixels 211, and a signal for driving theplurality of pixels 211 is supplied. In this specification and the like,in some cases, “a peripheral circuit” or “a driver circuit” indicatesall of the peripheral circuits 260, 270, 280, and 290. For example, theperipheral circuit 260 can be regarded as part of the peripheralcircuit.

The imaging device 200 preferably includes a light source 291. The lightsource 291 can emit detection light P1.

The peripheral circuit includes at least one of a logic circuit, aswitch, a buffer, an amplifier circuit, and a converter circuit. Theperipheral circuit may be provided over a substrate where the pixelportion 210 is formed. Part or the whole of the peripheral circuit maybe mounted using a semiconductor device such as an IC. Note that as theperipheral circuit, one or more of the peripheral circuits 260, 270,280, and 290 may be omitted.

As illustrated in FIG. 40B, the pixels 211 may be provided to beinclined in the pixel portion 210 included in the imaging device 200.When the pixels 211 are obliquely arranged, the distance between pixels(pitch) can be shortened in the row direction and the column direction.Accordingly, the quality of an image taken with the imaging device 200can be improved.

<Configuration Example 1 of Pixel>

The pixel 211 included in the imaging device 200 is formed with aplurality of subpixels 212, and each subpixel 212 is combined with afilter which transmits light with a specific wavelength range (colorfilter), whereby data for achieving color image display can be obtained.

FIG. 41A is a plan view illustrating an example of the pixel 211 withwhich a color image is obtained. The pixel 211 illustrated in FIG. 41Aincludes a subpixel 212 provided with a color filter transmitting lightwith a red (R) wavelength band (also referred to as a subpixel 212R), asubpixel 212 provided with a color filter transmitting light with agreen (G) wavelength band (also referred to as a subpixel 212G), and asubpixel 212 provided with a color filter transmitting light with a blue(B) wavelength band (also referred to as a subpixel 212B). The subpixel212 can function as a photosensor.

The subpixel 212 (the subpixel 212R, the subpixel 212G, and the subpixel212B) is electrically connected to a wiring 231, a wiring 247, a wiring248, a wiring 249, and a wiring 250. In addition, the subpixel 212R, thesubpixel 212G, and the subpixel 212B are connected to respective wirings253 which are independent from one another. In this specification andthe like, for example, the wiring 248 and the wiring 249 that areconnected to the pixel 211 in the n-th row (n is an integer greater thanor equal to 1 and less than or equal to p) are referred to as a wiring248[n] and a wiring 249[n]. For example, the wiring 253 connected to thepixel 211 in the m-th column (m is an integer greater than or equal to 1and less than or equal to q) is referred to as a wiring 253[m]. Notethat in FIG. 41A, the wirings 253 connected to the subpixel 212R, thesubpixel 212G, and the subpixel 212B in the pixel 211 in the m-th columnare referred to as a wiring 253[m]R, a wiring 253[m]G, and a wiring253[m]B, respectively. The subpixels 212 are electrically connected tothe peripheral circuit through the above wirings.

The imaging device 200 has a structure in which the subpixel 212 iselectrically connected to the subpixel 212 in an adjacent pixel 211which is provided with a color filter transmitting light with the samewavelength band as the subpixel 212, via a switch. FIG. 41B shows aconnection example of the subpixels 212: the subpixel 212 in the pixel211 arranged in an n-th row and an m-th column and the subpixel 212 inthe adjacent pixel 211 arranged in an (n+1)-th row and the m-th column.In FIG. 41B, the subpixel 212R arranged in the n-th row and the m-thcolumn and the subpixel 212R arranged in the (n+1)-th row and the m-thcolumn are connected to each other via a switch 201. The subpixel 212Garranged in the n-th row and the m-th column and the subpixel 212Garranged in the (n+1)-th row and the m-th column are connected to eachother via a switch 202. The subpixel 212B arranged in the n-th row andthe m-th column and the subpixel 212B arranged in the (n+1)-th row andthe m-th column are connected to each other via a switch 203.

The color filter used in the subpixel 212 is not limited to red (R),green (G), and blue (B) color filters, and color filters that transmitlight of cyan (C), yellow (Y), and magenta (M) may be used. By provisionof the subpixels 212 that sense light with three different wavelengthbands in one pixel 211, a full-color image can be obtained.

The pixel 211 including the subpixel 212 provided with a color filtertransmitting yellow (Y) light may be provided, in addition to thesubpixels 212 provided with the color filters transmitting red (R),green (G), and blue (B) light. The pixel 211 including the subpixel 212provided with a color filter transmitting blue (B) light may beprovided, in addition to the subpixels 212 provided with the colorfilters transmitting cyan (C), yellow (Y), and magenta (M) light. Whenthe subpixels 212 sensing light with four different wavelength bands areprovided in one pixel 211, the reproducibility of colors of an obtainedimage can be further increased.

For example, in FIG. 41A, in regard to the subpixel 212 sensing a redwavelength band, the subpixel 212 sensing a green wavelength band, andthe subpixel 212 sensing a blue wavelength band, the pixel number ratio(or the light receiving area ratio) thereof is not necessarily 1:1:1.For example, the Bayer arrangement in which the pixel number ratio (thelight receiving area ratio) of red to green and blue is 1:2:1 may beemployed. Alternatively, the pixel number ratio (the light receivingarea ratio) of red to green and blue may be 1:6:1.

Although the number of subpixels 212 provided in the pixel 211 may beone, two or more subpixels are preferably provided. For example, whentwo or more subpixels 212 sensing the same wavelength band are provided,the redundancy is increased, and the reliability of the imaging device200 can be increased.

When an infrared (IR) filter that transmits infrared light and absorbsor reflects visible light is used as the filter, the imaging device 200that senses infrared light can be achieved.

Furthermore, when a neutral density (ND) filter (dark filter) is used,output saturation which occurs when a large amount of light enters aphotoelectric conversion element (light-receiving element) can beprevented. With a combination of ND filters with different dimmingcapabilities, the dynamic range of the imaging device can be increased.

Besides the above-described filter, the pixel 211 may be provided with alens. An arrangement example of the pixel 211, a filter 254, and a lens255 is described with cross-sectional views in FIGS. 42A and 42B. Withthe lens 255, the photoelectric conversion element can receive incidentlight efficiently. Specifically, as illustrated in FIG. 42A, light 256enters a photoelectric conversion element 220 through the lens 255, thefilter 254 (a filter 254R, a filter 254G, and a filter 254B), a pixelcircuit 230, and the like which are provided in the pixel 211.

However, as indicated by a region surrounded with dashed-dotted lines,part of the light 256 indicated by arrows might be blocked by somewirings 257. Thus, a preferable structure is that the lens 255 and thefilter 254 are provided on the photoelectric conversion element 220side, so that the photoelectric conversion element 220 can efficientlyreceive the light 256 as illustrated in FIG. 42B. When the light 256enters the photoelectric conversion element 220 from the photoelectricconversion element 220 side, the imaging device 200 with highsensitivity can be provided.

As the photoelectric conversion element 220 illustrated in FIGS. 42A and42B, a photoelectric conversion element in which a p-n junction or ap-i-n junction is formed may be used.

The photoelectric conversion element 220 may be formed using a substancethat has a function of absorbing a radiation and generating electriccharges. Examples of the substance that has a function of absorbing aradiation and generating electric charges include selenium, lead iodide,mercury iodide, gallium arsenide, cadmium telluride, and a cadmium zincalloy.

For example, when selenium is used for the photoelectric conversionelement 220, the photoelectric conversion element 220 can have a lightabsorption coefficient in a wide wavelength band, such as visible light,ultraviolet light, infrared light, X-rays, and gamma rays.

One pixel 211 included in the imaging device 200 may include thesubpixel 212 with a first filter in addition to the subpixel 212illustrated in FIGS. 41A and 41B.

<Configuration Example 2 of Pixel>

An example of a pixel that includes a transistor including silicon and atransistor including an oxide semiconductor is described below.

FIGS. 43A and 43B are each a cross-sectional view of an element includedin an imaging device.

The imaging device illustrated in FIG. 43A includes a transistor 351including silicon over a silicon substrate 300, transistors 352 and 353which include an oxide semiconductor and are stacked over the transistor351, and a photodiode 360 provided in the silicon substrate 300 andincluding an anode 361 and a cathode 362. The transistors and thephotodiode 360 are electrically connected to various plugs 370 andwirings 371. In addition, the anode 361 of the photodiode 360 iselectrically connected to the plug 370 through a low-resistance region363.

The imaging device includes a layer 310 including the transistor 351provided on the silicon substrate 300 and the photodiode 360 provided inthe silicon substrate 300, a layer 320 which is in contact with thelayer 310 and includes the wirings 371, a layer 330 which is in contactwith the layer 320 and includes the transistors 352 and 353, and a layer340 which is in contact with the layer 330 and includes a wiring 372 anda wiring 373.

Note that in the example of a cross-sectional view in FIG. 43A, alight-receiving surface of the photodiode 360 is provided on the sideopposite to a surface of the silicon substrate 300 where the transistor351 is formed. With the structure, an optical path can be obtainedwithout the influence by the transistors or wirings, and therefore, apixel with a high aperture ratio can be formed. Note that thelight-receiving surface of the photodiode 360 can be the same as thesurface where the transistor 351 is formed.

In the case of forming a pixel with the use of transistors including anoxide semiconductor, the layer 310 may include the transistor includingan oxide semiconductor. Alternatively, the layer 310 may be omitted, andthe pixel may include only transistors including an oxide semiconductor.

In addition, in the cross-sectional view in FIG. 43A, the photodiode 360in the layer 310 and the transistor in the layer 330 can be formed so asto overlap with each other. Thus, the degree of integration of pixelscan be increased. In other words, the resolution of the imaging devicecan be increased.

An imaging device illustrated in FIG. 43B includes a photodiode 365 inthe layer 340 and over the transistor. In FIG. 43B, the layer 310includes the transistor 351 and a transistor 352 which include silicon,the layer 320 includes the wiring 371, the layer 330 includes thetransistors 352 and 353 which include an oxide semiconductor, and thelayer 340 includes the photodiode 365. The photodiode 365 includes asemiconductor layer 366, a semiconductor layer 367, and a semiconductorlayer 368, and is electrically connected to the wiring 373, and to awiring 374 through the plug 370.

The element structure illustrated in FIG. 43B can increase the apertureratio.

Alternatively, a PIN diode element formed using an amorphous siliconfilm, a microcrystalline silicon film, or the like may be used as thephotodiode 365. In the photodiode 365, an n-type semiconductor layer368, an i-type semiconductor layer 367, and a p-type semiconductor layer366 are stacked in this order. The i-type semiconductor layer 367 ispreferably formed using amorphous silicon. The p-type semiconductorlayer 366 and the n-type semiconductor layer 368 can each be formedusing amorphous silicon, microcrystalline silicon, or the like whichincludes a dopant imparting the corresponding conductivity type. Thephotodiode 365 in which a photoelectric conversion layer is formed usingamorphous silicon has high sensitivity in a visible light wavelengthregion, and therefore can easily sense weak visible light.

This embodiment can be combined as appropriate with any of the otherembodiments in this specification.

Embodiment 5

<RF Tag>

In this embodiment, an RF tag that includes the transistor described inthe above embodiments or the memory device described in the aboveembodiment will be described with reference to FIG. 44.

The RF tag of this embodiment includes a memory circuit, storesnecessary data in the memory circuit, and transmits and receives datato/from the outside by using contactless means, for example, wirelesscommunication. With these features, the RF tag can be used for anindividual authentication system in which an object or the like isrecognized by reading the individual information, for example. Note thatthe RF tag is required to have extremely high reliability in order to beused for this purpose.

A configuration of the RF tag is described with reference to FIG. 44.FIG. 44 is a block diagram illustrating a configuration example of an RFtag.

As illustrated in FIG. 44, an RF tag 800 includes an antenna 804 whichreceives a radio signal 803 that is transmitted from an antenna 802connected to a communication device 801 (also referred to as aninterrogator, a reader/writer, or the like). The RF tag 800 includes arectifier circuit 805, a constant voltage circuit 806, a demodulationcircuit 807, a modulation circuit 808, a logic circuit 809, a memorycircuit 810, and a ROM 811. A transistor having a rectifying functionincluded in the demodulation circuit 807 may be formed using a materialwhich enables a reverse current to be low enough, for example, an oxidesemiconductor. This can suppress the phenomenon of a rectifying functionbecoming weaker due to generation of a reverse current and preventsaturation of the output from the demodulation circuit. In other words,the input to the demodulation circuit and the output from thedemodulation circuit can have a relation closer to a linear relation.Note that data transmission methods are roughly classified into thefollowing three methods: an electromagnetic coupling method in which apair of coils is provided so as to face each other and communicates witheach other by mutual induction, an electromagnetic induction method inwhich communication is performed using an induction field, and a radiowave method in which communication is performed using a radio wave. Anyof these methods can be used in the RF tag 800 described in thisembodiment.

Next, the structure of each circuit is described. The antenna 804exchanges the radio signal 803 with the antenna 802 which is connectedto the communication device 801. The rectifier circuit 805 generates aninput potential by rectification, for example, half-wave voltage doublerrectification of an input alternating signal generated by reception of aradio signal at the antenna 804 and smoothing of the rectified signalwith a capacitor provided in a later stage in the rectifier circuit 805.Note that a limiter circuit may be provided on an input side or anoutput side of the rectifier circuit 805. The limiter circuit controlselectric power so that electric power which is higher than or equal tocertain electric power is not input to a circuit in a later stage if theamplitude of the input alternating signal is high and an internalgeneration voltage is high.

The constant voltage circuit 806 generates a stable power supply voltagefrom an input potential and supplies it to each circuit. Note that theconstant voltage circuit 806 may include a reset signal generationcircuit. The reset signal generation circuit generates a reset signal ofthe logic circuit 809 by utilizing rise of the stable power supplyvoltage.

The demodulation circuit 807 demodulates the input alternating signal byenvelope detection and generates the demodulated signal. Further, themodulation circuit 808 performs modulation in accordance with data to beoutput from the antenna 804.

The logic circuit 809 analyzes and processes the demodulated signal. Thememory circuit 810 holds the input data and includes a row decoder, acolumn decoder, a memory region, and the like. Further, the ROM 811stores an identification number (ID) or the like and outputs it inaccordance with processing.

Note that the decision whether each circuit described above is providedor not can be made as appropriate.

Here, the transistor described in the above embodiment can be used asthe memory circuit 810. Since the transistor of one embodiment of thepresent invention can retain data even when not powered, the memorycircuit can be favorably used for an RF tag. Furthermore, the memorycircuit of one embodiment of the present invention needs power (voltage)needed for data writing significantly lower than that needed in aconventional nonvolatile memory; thus, it is possible to prevent adifference between the maximum communication range in data reading andthat in data writing. In addition, it is possible to suppressmalfunction or incorrect writing which is caused by power shortage indata writing.

Since the memory circuit of one embodiment of the present invention canbe used as a nonvolatile memory, it can also be used as the ROM 811. Inthis case, it is preferable that a manufacturer separately prepare acommand for writing data to the ROM 811 so that a user cannot rewritedata freely. Since the manufacturer gives identification numbers beforeshipment and then starts shipment of products, instead of puttingidentification numbers to all the manufactured RF tags, it is possibleto put identification numbers to only good products to be shipped. Thus,the identification numbers of the shipped products are in series andcustomer management corresponding to the shipped products is easilyperformed.

This embodiment can be combined as appropriate with any of the otherembodiments in this specification.

Embodiment 6

In this embodiment, a CPU that includes the memory device described inthe above embodiment will be described.

FIG. 45 is a block diagram illustrating a configuration example of a CPUat least partly including any of the transistors described in the aboveembodiments as a component.

<CPU>

The CPU illustrated in FIG. 45 includes, over a substrate 1190, anarithmetic logic unit (ALU) 1191, an ALU controller 1192, an instructiondecoder 1193, an interrupt controller 1194, a timing controller 1195, aregister 1196, a register controller 1197, a bus interface 1198, arewritable ROM 1199, and a ROM interface 1189. A semiconductorsubstrate, an SOI substrate, a glass substrate, or the like is used asthe substrate 1190. The ROM 1199 and the ROM interface 1189 may beprovided over a separate chip. Needless to say, the CPU in FIG. 45 isjust an example in which the configuration is simplified, and an actualCPU may have a variety of configurations depending on the application.For example, the CPU may have the following configuration: a structureincluding the CPU illustrated in FIG. 45 or an arithmetic circuit isconsidered as one core; a plurality of the cores are included; and thecores operate in parallel. The number of bits that the CPU can processin an internal arithmetic circuit or in a data bus can be 8, 16, 32, or64, for example.

An instruction that is input to the CPU through the bus interface 1198is input to the instruction decoder 1193 and decoded therein, and then,input to the ALU controller 1192, the interrupt controller 1194, theregister controller 1197, and the timing controller 1195.

The ALU controller 1192, the interrupt controller 1194, the registercontroller 1197, and the timing controller 1195 conduct various controlsin accordance with the decoded instruction. Specifically, the ALUcontroller 1192 generates signals for controlling the operation of theALU 1191. While the CPU is executing a program, the interrupt controller1194 judges an interrupt request from an external input/output device ora peripheral circuit on the basis of its priority or a mask state, andprocesses the request. The register controller 1197 generates an addressof the register 1196, and reads/writes data from/to the register 1196 inaccordance with the state of the CPU.

The timing controller 1195 generates signals for controlling operationtimings of the ALU 1191, the ALU controller 1192, the instructiondecoder 1193, the interrupt controller 1194, and the register controller1197. For example, the timing controller 1195 includes an internal clockgenerator for generating an internal clock signal based on a referenceclock signal, and supplies the internal clock signal to the abovecircuits.

In the CPU illustrated in FIG. 45, a memory cell is provided in theregister 1196. For the memory cell of the register 1196, any of thetransistors described in Embodiments 1 to 3 can be used.

In the CPU illustrated in FIG. 45, the register controller 1197 selectsoperation of retaining data in the register 1196 in accordance with aninstruction from the ALU 1191. That is, the register controller 1197selects whether data is retained by a flip-flop or by a capacitor in thememory cell included in the register 1196. When data retaining by theflip-flop is selected, a power supply voltage is supplied to the memorycell in the register 1196. When data retaining by the capacitor isselected, the data is rewritten in the capacitor, and supply of powersupply voltage to the memory cell in the register 1196 can be stopped.

<Memory Circuit>

FIG. 46 is an example of a circuit diagram of a memory element that canbe used as the register 1196. A memory element 1200 includes a circuit1201 in which stored data is volatile when power supply is stopped, acircuit 1202 in which stored data is nonvolatile even when power supplyis stopped, a switch 1203, a switch 1204, a logic element 1206, acapacitor 1207, and a circuit 1220 having a selecting function. Thecircuit 1202 includes a capacitor 1208, a transistor 1209, and atransistor 1210. Note that the memory element 1200 may further includeanother element such as a diode, a resistor, or an inductor, as needed.

Here, the memory device described in the above embodiment can be used asthe circuit 1202. When supply of a power supply voltage to the memoryelement 1200 is stopped, a ground potential (0 V) or a potential atwhich the transistor 1209 in the circuit 1202 is turned off continues tobe input to a gate of the transistor 1209. For example, a first gate ofthe transistor 1209 is grounded through a load such as a resistor.

Shown here is an example in which the switch 1203 is a transistor 1213having one conductivity type (e.g., an n-channel transistor) and theswitch 1204 is a transistor 1214 having a conductivity type opposite tothe one conductivity type (e.g., a p-channel transistor). A firstterminal of the switch 1203 corresponds to one of a source electrode anda drain electrode of the transistor 1213, a second terminal of theswitch 1203 corresponds to the other of the source electrode and thedrain electrode of the transistor 1213, and conduction or non-conductionbetween the first terminal and the second terminal of the switch 1203(i.e., the on/off state of the transistor 1213) is selected by a controlsignal RD input to a gate of the transistor 1213. A first terminal ofthe switch 1204 corresponds to one of a source electrode and a drainelectrode of the transistor 1214, a second terminal of the switch 1204corresponds to the other of the source electrode and the drain electrodeof the transistor 1214, and conduction or non-conduction between thefirst terminal and the second terminal of the switch 1204 (i.e., theon/off state of the transistor 1214) is selected by the control signalRD input to a gate of the transistor 1214.

One of a source electrode and a drain electrode of the transistor 1209is electrically connected to one of a pair of electrodes of thecapacitor 1208 and a gate of the transistor 1210. Here, the connectionportion is referred to as a node M2. One of a source electrode and adrain electrode of the transistor 1210 is electrically connected to aline which can supply a low power supply potential (e.g., a GND line),and the other thereof is electrically connected to the first terminal ofthe switch 1203 (the one of the source electrode and the drain electrodeof the transistor 1213). The second terminal of the switch 1203 (theother of the source electrode and the drain electrode of the transistor1213) is electrically connected to the first terminal of the switch 1204(the one of the source electrode and the drain electrode of thetransistor 1214). The second terminal of the switch 1204 (the other ofthe source electrode and the drain electrode of the transistor 1214) iselectrically connected to a line which can supply a power supplypotential VDD. The second terminal of the switch 1203 (the other of thesource electrode and the drain electrode of the transistor 1213), thefirst terminal of the switch 1204 (the one of the source electrode andthe drain electrode of the transistor 1214), an input terminal of thelogic element 1206, and one of a pair of electrodes of the capacitor1207 are electrically connected to one another. Here, the connectionportion is referred to as a node M1. The other of the pair of electrodesof the capacitor 1207 can be supplied with a constant potential. Forexample, the other of the pair of electrodes of the capacitor 1207 canbe supplied with a low power supply potential (e.g., GND) or a highpower supply potential (e.g., VDD). The other of the pair of electrodesof the capacitor 1207 is electrically connected to the line which cansupply a low power supply potential (e.g., a GND line). The other of thepair of electrodes of the capacitor 1208 can be supplied with a constantpotential. For example, the other of the pair of electrodes of thecapacitor 1208 can be supplied with a low power supply potential (e.g.,GND) or a high power supply potential (e.g., VDD). The other of the pairof electrodes of the capacitor 1208 is electrically connected to theline which can supply a low power supply potential (e.g., a GND line).

The capacitor 1207 and the capacitor 1208 are not necessarily providedas long as the parasitic capacitance of the transistor, the wiring, orthe like is actively utilized.

A control signal WE is input to the first gate (first gate electrode) ofthe transistor 1209. As for each of the switch 1203 and the switch 1204,a conduction state or a non-conduction state between the first terminaland the second terminal is selected by the control signal RD which isdifferent from the control signal WE. When the first terminal and thesecond terminal of one of the switches are in the conduction state, thefirst terminal and the second terminal of the other of the switches arein the non-conduction state.

Note that the transistor 1209 in FIG. 46 has a structure with a secondgate (second gate electrode: back gate). The control signal WE can beinput to the first gate and the control signal WE2 can be input to thesecond gate. The control signal WE2 is a signal having a constantpotential. As the constant potential, for example, a ground potentialGND or a potential lower than a potential of the source electrode of thetransistor 1209 is selected. The control signal WE2 is a potentialsignal for controlling the threshold voltage of the transistor 1209, anda current when a gate voltage is 0 V can be further reduced. The controlsignal WE2 may be a signal having the same potential as that of thecontrol signal WE. Note that as the transistor 1209, a transistorwithout a second gate may be used.

A signal corresponding to data retained in the circuit 1201 is input tothe other of the source electrode and the drain electrode of thetransistor 1209. FIG. 46 illustrates an example in which a signal outputfrom the circuit 1201 is input to the other of the source electrode andthe drain electrode of the transistor 1209. The logic value of a signaloutput from the second terminal of the switch 1203 (the other of thesource electrode and the drain electrode of the transistor 1213) isinverted by the logic element 1206, and the inverted signal is input tothe circuit 1201 through the circuit 1220.

In the example of FIG. 46, a signal output from the second terminal ofthe switch 1203 (the other of the source electrode and the drainelectrode of the transistor 1213) is input to the circuit 1201 throughthe logic element 1206 and the circuit 1220; however, one embodiment ofthe present invention is not limited thereto. The signal output from thesecond terminal of the switch 1203 (the other of the source electrodeand the drain electrode of the transistor 1213) may be input to thecircuit 1201 without its logic value being inverted. For example, in thecase where the circuit 1201 includes a node in which a signal obtainedby inversion of the logic value of a signal input from the inputterminal is retained, the signal output from the second terminal of theswitch 1203 (the other of the source electrode and the drain electrodeof the transistor 1213) can be input to the node.

In FIG. 46, the transistors included in the memory element 1200 exceptfor the transistor 1209 can each be a transistor in which a channel isformed in a layer formed using a semiconductor other than an oxidesemiconductor or in the substrate 1190. For example, the transistor canbe a transistor whose channel is formed in a silicon layer or a siliconsubstrate. Alternatively, all the transistors in the memory element 1200may be a transistor in which a channel is formed in an oxidesemiconductor layer. Further alternatively, in the memory element 1200,a transistor in which a channel is formed in an oxide semiconductorlayer can be included besides the transistor 1209, and a transistor inwhich a channel is formed in a layer including a semiconductor otherthan an oxide semiconductor or in the substrate 1190 can be used for therest of the transistors.

As the circuit 1201 in FIG. 46, for example, a flip-flop circuit can beused. As the logic element 1206, for example, an inverter or a clockedinverter can be used.

In a period during which the memory element 1200 is not supplied withthe power supply voltage, the semiconductor device of one embodiment ofthe present invention can retain data stored in the circuit 1201 by thecapacitor 1208 which is provided in the circuit 1202.

The off-state current of a transistor in which a channel is formed in anoxide semiconductor layer is extremely low. For example, the off-statecurrent of a transistor in which a channel is formed in an oxidesemiconductor layer is significantly lower than that of a transistor inwhich a channel is formed in silicon having crystallinity. Thus, whenthe transistor is used as the transistor 1209, a signal held in thecapacitor 1208 is retained for a long time also in a period during whichthe power supply voltage is not supplied to the memory element 1200. Thememory element 1200 can accordingly retain the stored content (data)also in a period during which the supply of the power supply voltage isstopped.

Since the above-described memory element performs pre-charge operationwith the switch 1203 and the switch 1204, the time required for thecircuit 1201 to retain original data again after the supply of the powersupply voltage is restarted can be shortened.

In the circuit 1202, a signal retained by the capacitor 1208 is input tothe gate of the transistor 1210. Therefore, after supply of the powersupply voltage to the memory element 1200 is restarted, the signalretained by the capacitor 1208 can be converted into the onecorresponding to the state (the on state or the off state) of thetransistor 1210 to be read from the circuit 1202. Consequently, anoriginal signal can be accurately read even when a potentialcorresponding to the signal retained by the capacitor 1208 varies tosome degree.

By applying the above-described memory element 1200 to a memory devicesuch as a register or a cache memory included in a processor, data inthe memory device can be prevented from being lost owing to the stop ofthe supply of the power supply voltage. Furthermore, shortly after thesupply of the power supply voltage is restarted, the memory device canbe returned to the same state as that before the power supply isstopped. Therefore, the power supply can be stopped even for a shorttime in the processor or one or a plurality of logic circuits includedin the processor, resulting in lower power consumption.

Although the memory element 1200 is used in a CPU in this embodiment,the memory element 1200 can also be used in an LSI such as a digitalsignal processor (DSP), a custom LSI, or a programmable logic device(PLD), and a radio frequency (RF) tag.

This embodiment can be combined as appropriate with any of the otherembodiments in this specification.

Embodiment 7

In this embodiment, configuration examples of a display device using atransistor of one embodiment of the present invention will be described.

<Circuit Configuration Example of Display Device>

FIG. 47A is a top view of the display device of one embodiment of thepresent invention. FIG. 47B is a circuit diagram illustrating a pixelcircuit that can be used in the case where a liquid crystal element isused in a pixel in the display device of one embodiment of the presentinvention. FIG. 47C is a circuit diagram illustrating a pixel circuitthat can be used in the case where an organic EL element is used in apixel in the display device of one embodiment of the present invention.

The transistor in the pixel portion can be formed in accordance withEmbodiments 1 to 3. The transistor can be easily formed as an n-channeltransistor, and thus part of a driver circuit that can be formed usingan n-channel transistor can be formed over the same substrate as thetransistor of the pixel portion. With the use of any of the transistorsdescribed in the above embodiments for the pixel portion or the drivercircuit in this manner, a highly reliable display device can beprovided.

FIG. 47A illustrates an example of a top view of an active matrixdisplay device. A pixel portion 701, a first scan line driver circuit702, a second scan line driver circuit 703, and a signal line drivercircuit 704 are formed over a substrate 700 of the display device. Inthe pixel portion 701, a plurality of signal lines extended from thesignal line driver circuit 704 are arranged and a plurality of scanlines extended from the first scan line driver circuit 702 and thesecond scan line driver circuit 703 are arranged. Note that pixels whichinclude display elements are provided in a matrix in respective regionswhere the scan lines and the signal lines intersect with each other. Thesubstrate 700 of the display device is connected to a timing controlcircuit (also referred to as a controller or a controller IC) through aconnection portion such as a flexible printed circuit (FPC).

In FIG. 47A, the first scan line driver circuit 702, the second scanline driver circuit 703, and the signal line driver circuit 704 areformed over the substrate 700 where the pixel portion 701 is formed.Accordingly, the number of components which are provided outside, suchas a driver circuit, can be reduced, so that a reduction in cost can beachieved. Furthermore, if the driver circuit is provided outside thesubstrate 700, wirings would need to be extended and the number ofwiring connections would increase. When the driver circuit is providedover the substrate 700, the number of wiring connections can be reduced.Consequently, an improvement in reliability or yield can be achieved.One or more of the first scan line driver circuit 702, the second scanline driver circuit 703, and the signal line driver circuit 704 may bemounted on the substrate 700 or provided outside the substrate 700.

<Liquid Crystal Display Device>

FIG. 47B illustrates an example of a circuit configuration of the pixel.Here, a pixel circuit which is applicable to a pixel of a VA liquidcrystal display device is illustrated as an example.

This pixel circuit can be applied to a structure in which one pixelincludes a plurality of pixel electrode layers. The pixel electrodelayers are connected to different transistors, and the transistors canbe driven with different gate signals. Accordingly, signals applied toindividual pixel electrode layers in a multi-domain pixel can becontrolled independently.

A scan line 712 of a transistor 716 and a scan line 713 of a transistor717 are separated so that different gate signals can be suppliedthereto. In contrast, a signal line 714 is shared by the transistors 716and 717. The transistor described in any of Embodiments 1 to 3 can beused as appropriate as each of the transistors 716 and 717. Thus, ahighly reliable liquid crystal display device can be provided.

A first pixel electrode layer is electrically connected to thetransistor 716 and a second pixel electrode layer is electricallyconnected to the transistor 717. The first pixel electrode layer and thesecond pixel electrode layer are separated. Shapes of the first pixelelectrode layer and the second pixel electrode layer are notparticularly limited. For example, the first pixel electrode layer mayhave a V-like shape.

A gate electrode of the transistor 716 is connected to the scan line712, and a gate electrode of the transistor 717 is connected to the scanline 713. When different gate signals are supplied to the scan line 712and the scan line 713, operation timings of the transistor 716 and thetransistor 717 can be varied. As a result, alignment of liquid crystalscan be controlled.

Further, a storage capacitor may be formed using a capacitor wiring 710,a gate insulating layer functioning as a dielectric, and a capacitorelectrode electrically connected to the first pixel electrode layer orthe second pixel electrode layer.

The multi-domain pixel includes a first liquid crystal element 718 and asecond liquid crystal element 719. The first liquid crystal element 718includes the first pixel electrode layer, a counter electrode layer, anda liquid crystal layer therebetween. The second liquid crystal element719 includes the second pixel electrode layer, a counter electrodelayer, and a liquid crystal layer therebetween.

Note that a pixel circuit of the present invention is not limited tothat illustrated in FIG. 47B. For example, a switch, a resistor, acapacitor, a transistor, a sensor, a logic circuit, or the like may beadded to the pixel circuit illustrated in FIG. 47B.

FIGS. 48A and 48B are examples of a top view and a cross-sectional viewof a liquid crystal display device. Note that FIG. 48A illustrates atypical structure including a display device 20, a display region 21, aperipheral circuit 22, and a flexible printed circuit (FPC) 42.

FIG. 48B is a cross-sectional view taken along the dashed lines A-A′,B-B′, and C-C′ in FIG. 48A. The cross section taken along the dashedline A-A′ illustrates the peripheral circuit portion, the cross sectiontaken along the dashed line B-B′ illustrates the display region, and thecross section taken along the dashed line C-C′ illustrates a portionconnected to the FPC.

The display device 20 includes the following in addition to thetransistor 11: a conductive layer 190, a conductive layer 195, aninsulating layer 420, a liquid crystal layer 490, a liquid crystalelement 80, a capacitor 60, an insulating layer 430, a spacer 440, acoloring layer 460, a bonding layer 470, a conductive layer 480, alight-shielding layer 418, a substrate 400, a bonding layer 473, abonding layer 474, a bonding layer 475, a bonding layer 476, apolarizing plate 103, a polarizing plate 403, a protective substrate105, a protective substrate 402, and an anisotropic conductive layer510.

<Organic EL Display Device>

FIG. 47C illustrates another example of a circuit configuration of thepixel. Here, a pixel structure of a display device using an organic ELelement is shown.

In an organic EL element, by application of voltage to a light-emittingelement, electrons are injected from one of a pair of electrodes andholes are injected from the other of the pair of electrodes, into alayer containing a light-emitting organic compound; thus, current flows.The electrons and holes are recombined, and thus, the light-emittingorganic compound is excited. The light-emitting organic compound returnsto a ground state from the excited state, thereby emitting light. Owingto such a mechanism, this light-emitting element is referred to as acurrent-excitation light-emitting element.

FIG. 47C illustrates an applicable example of a pixel circuit. Here, onepixel includes two n-channel transistors. Furthermore, digital timegrayscale driving can be employed for the pixel circuit.

The configuration of the applicable pixel circuit and operation of apixel employing digital time grayscale driving are described.

A pixel 720 includes a switching transistor 721, a driver transistor722, a light-emitting element 724, and a capacitor 723. A gate electrodelayer of the switching transistor 721 is connected to a scan line 726, afirst electrode (one of a source electrode layer and a drain electrodelayer) of the switching transistor 721 is connected to a signal line725, and a second electrode (the other of the source electrode layer andthe drain electrode layer) of the switching transistor 721 is connectedto a gate electrode layer of the driver transistor 722. The gateelectrode layer of the driver transistor 722 is connected to a powersupply line 727 through the capacitor 723, a first electrode of thedriver transistor 722 is connected to the power supply line 727, and asecond electrode of the driver transistor 722 is connected to a firstelectrode (a pixel electrode) of the light-emitting element 724. Asecond electrode of the light-emitting element 724 corresponds to acommon electrode 728. The common electrode 728 is electrically connectedto a common potential line formed over the same substrate as the commonelectrode 728.

As the switching transistor 721 and the driver transistor 722, thetransistor described in any of Embodiments 1 to 3 can be used asappropriate. In this manner, a highly reliable organic EL display devicecan be provided.

The potential of the second electrode (the common electrode 728) of thelight-emitting element 724 is set to be a low power supply potential.Note that the low power supply potential is lower than a high powersupply potential supplied to the power supply line 727. For example, thelow power supply potential can be GND, 0V, or the like. The high powersupply potential and the low power supply potential are set to be higherthan or equal to the forward threshold voltage of the light-emittingelement 724, and the difference between the potentials is applied to thelight-emitting element 724, whereby current is supplied to thelight-emitting element 724, leading to light emission. The forwardvoltage of the light-emitting element 724 refers to a voltage at which adesired luminance is obtained, and includes at least a forward thresholdvoltage.

Note that gate capacitance of the driver transistor 722 may be used as asubstitute for the capacitor 723, so that the capacitor 723 can beomitted.

Next, a signal input to the driver transistor 722 is described. In thecase of a voltage-input voltage driving method, a video signal forsufficiently turning on or off the driver transistor 722 is input to thedriver transistor 722. In order for the driver transistor 722 to operatein a linear region, voltage higher than the voltage of the power supplyline 727 is applied to the gate electrode layer of the driver transistor722. Note that voltage higher than or equal to voltage which is the sumof power supply line voltage and the threshold voltage Vth of the drivertransistor 722 is applied to the signal line 725.

In the case of performing analog grayscale driving, a voltage greaterthan or equal to a voltage which is the sum of the forward voltage ofthe light-emitting element 724 and the threshold voltage Vth of thedriver transistor 722 is applied to the gate electrode layer of thedriver transistor 722. A video signal by which the driver transistor 722is operated in a saturation region is input, so that current is suppliedto the light-emitting element 724. In order for the driver transistor722 to operate in a saturation region, the potential of the power supplyline 727 is set higher than the gate potential of the driver transistor722. When an analog video signal is used, it is possible to supplycurrent to the light-emitting element 724 in accordance with the videosignal and perform analog grayscale driving.

Note that the configuration of the pixel circuit of the presentinvention is not limited to that illustrated in FIG. 47C. For example, aswitch, a resistor, a capacitor, a sensor, a transistor, a logiccircuit, or the like may be added to the pixel circuit illustrated inFIG. 47C.

In the case where the transistor shown in any of the above embodimentsis used for the circuit shown in FIGS. 47A to 47C, the source electrode(the first electrode) is electrically connected to the low potentialside and the drain electrode (the second electrode) is electricallyconnected to the high potential side. Furthermore, the potential of thefirst gate electrode may be controlled by a control circuit or the likeand the potential described above as an example, for example, apotential lower than the potential applied to the source electrode, maybe input to the second gate electrode through a wiring that is notillustrated.

FIGS. 49A and 49B are examples of a top view and a cross-sectional viewof a light-emitting device. Note that FIG. 49A illustrates a typicalstructure including the light-emitting device 24, the display region 21,the peripheral circuit 22, and the flexible printed circuit (FPC) 42.

FIG. 49B is a cross-sectional view taken along the dashed lines A-A′,B-B′, and C-C′ in FIG. 49A. The cross section taken along the dashedline A-A′ illustrates the peripheral circuit portion, the cross sectiontaken along the dashed line B-B′ illustrates the display region, and thecross section taken along the dashed line C-C′ illustrates a portionconnected to the FPC.

A light-emitting device 24 includes the following in addition to thetransistor 11: the conductive layer 190, the conductive layer 195, aconductive layer 410, an optical adjustment layer 530, an EL layer 450,a light-emitting element 70, the capacitor 60, the spacer 440, thecoloring layer 460, the bonding layer 470, the conductive layer 480, thelight-shielding layer 418, the substrate 400, and an anisotropicconductive layer 510.

For example, in this specification and the like, a display element, adisplay device which is a device including a display element, alight-emitting element, and a light-emitting device which is a deviceincluding a light-emitting element can employ a variety of modes or caninclude a variety of elements. A display element, a display device, alight-emitting element, or a light-emitting device include at least oneof the following, for example: an EL (electroluminescent) element (e.g.,an EL element including organic and inorganic materials, an organic ELelement, or an inorganic EL element), an LED (e.g., a white LED, a redLED, a green LED, or a blue LED), a transistor (a transistor which emitslight depending on current), an electron emitter, a liquid crystalelement, electronic ink, an electrophoretic element, a grating lightvalve (GLV), a plasma display panel (PDP), micro electro mechanicalsystems (MEMS), a digital micromirror device (DMD), a digital microshutter (DMS), MIRASOL (registered trademark), an interferometricmodulator display (IMOD) element, an electrowetting element, apiezoelectric ceramic display, and a display element using a carbonnanotube. Other than the above, display media whose contrast, luminance,reflectivity, transmittance, or the like is changed by electric orelectromagnetic action may be included. Note that examples of displaydevices having EL elements include an EL display. Examples of displaydevices including electron emitters include a field emission display(FED) and an SED-type flat panel display (SED: surface-conductionelectron-emitter display). Examples of display devices including liquidcrystal elements include a liquid crystal display (e.g., a transmissiveliquid crystal display, a transflective liquid crystal display, areflective liquid crystal display, a direct-view liquid crystal display,or a projection liquid crystal display). Examples of display devicesincluding electronic ink or electrophoretic elements include electronicpaper.

This embodiment can be combined as appropriate with any of the otherembodiments in this specification.

Embodiment 8

In this embodiment, a display module using a semiconductor device of oneembodiment of the present invention will be described with reference toFIG. 50.

<Display Module>

In a display module 6000 in FIG. 50, a touch panel 6004 connected to anFPC 6003, a display panel 6006 connected to an FPC 6005, a backlightunit 6007, a frame 6009, a printed board 6010, and a battery 6011 areprovided between an upper cover 6001 and a lower cover 6002. Note thatthe backlight unit 6007, the battery 6011, the touch panel 6004, and thelike are not provided in some cases.

The semiconductor device of one embodiment of the present invention canbe used for, for example, the display panel 6006 and an integratedcircuit mounted on a printed circuit board.

The shapes and sizes of the upper cover 6001 and the lower cover 6002can be changed as appropriate in accordance with the sizes of the touchpanel 6004 and the display panel 6006.

The touch panel 6004 can be a resistive touch panel or a capacitivetouch panel and may be formed to overlap with the display panel 6006. Acounter substrate (sealing substrate) of the display panel 6006 can havea touch panel function. A photosensor may be provided in each pixel ofthe display panel 6006 so that an optical touch panel function is added.An electrode for a touch sensor may be provided in each pixel of thedisplay panel 6006 so that a capacitive touch panel function is added.

The backlight unit 6007 includes a light source 6008. The light source6008 may be provided at an end portion of the backlight unit 6007 and alight diffusing plate may be used.

The frame 6009 protects the display panel 6006 and also functions as anelectromagnetic shield for blocking electromagnetic waves generated fromthe printed board 6010. The frame 6009 may function as a radiator plate.

The printed board 6010 has a power supply circuit and a signalprocessing circuit for outputting a video signal and a clock signal. Asa power source for supplying power to the power supply circuit, anexternal commercial power source or the battery 6011 provided separatelymay be used. Note that the battery 6011 is not necessary in the casewhere a commercial power source is used.

The display module 6000 can be additionally provided with a member suchas a polarizing plate, a retardation plate, or a prism sheet.

This embodiment can be combined as appropriate with any of the otherembodiments in this specification.

Embodiment 9

In this embodiment, application examples of the semiconductor device inone embodiment of the present invention will be described.

<Package Using Lead Frame Interposer>

FIG. 51A is a perspective view illustrating a cross-sectional structureof a package using a lead frame interposer. In the package illustratedin FIG. 51A, a chip 2751 corresponding to the semiconductor device ofone embodiment of the present invention is connected to a terminal 2752over an interposer 2750 by wire bonding. The terminal 2752 is placed ona surface of the interposer 2750 on which the chip 2751 is mounted. Thechip 2751 may be sealed by a mold resin 2753, in which case the chip2751 is sealed such that part of each of the terminals 2752 is exposed.

FIG. 51B illustrates the structure of a module of an electronic device(cellular phone) in which a package is mounted on a circuit board. Inthe module of the mobile phone in FIG. 51B, a package 2802 and a battery2804 are mounted on a printed wiring board 2801. The printed wiringboard 2801 is mounted on a panel 2800 including a display element by anFPC 2803.

This embodiment can be combined as appropriate with any of the otherembodiments in this specification.

Embodiment 10

In this embodiment, electronic devices and lighting devices of oneembodiment of the present invention will be described with reference todrawings.

<Electronic Device>

Electronic devices and lighting devices can be manufactured using thesemiconductor device of one embodiment of the present invention. Inaddition, highly reliable electronic devices and lighting devices can bemanufactured using the semiconductor device of one embodiment of thepresent invention. Furthermore, electronic devices and lighting devicesincluding touch sensors with improved detection sensitivity can bemanufactured using the semiconductor device of one embodiment of thepresent invention.

Examples of electronic devices are television devices (also referred toas TV or television receivers), monitors for computers and the like,cameras such as digital cameras and digital video cameras, digital photoframes, cellular phones (also referred to as portable telephonedevices), portable game machines, portable information terminals, audioplayback devices, large game machines such as pin-ball machines, and thelike.

In the case of having flexibility, the electronic device or lightingdevice of one embodiment of the present invention can be incorporatedalong a curved inside/outside wall surface of a house or a building or acurved interior/exterior surface of a car.

Furthermore, the electronic device of one embodiment of the presentinvention may include a secondary battery. It is preferable that thesecondary battery be capable of being charged by non-contact powertransmission.

Examples of the secondary battery include a lithium ion secondarybattery such as a lithium polymer battery using a gel electrolyte(lithium ion polymer battery), a nickel-hydride battery, anickel-cadmium battery, an organic radical battery, a lead-acid battery,an air secondary battery, a nickel-zinc battery, and a silver-zincbattery.

The electronic device of one embodiment of the present invention mayinclude an antenna. When a signal is received by the antenna, theelectronic device can display an image, data, or the like on a displayportion. When the electronic device includes a secondary battery, theantenna may be used for non-contact power transmission.

FIG. 52A illustrates a portable game machine, which includes a housing7101, a housing 7102, a display portion 7103, a display portion 7104, amicrophone 7105, speakers 7106, an operation key 7107, a stylus 7108,and the like. The semiconductor device of one embodiment of the presentinvention can be used for an integrated circuit, a CPU, or the likeincorporated in the housing 7101. When the display device of oneembodiment of the present invention is used as the display portion 7103or 7104, it is possible to provide a user-friendly portable game machinewith quality that hardly deteriorates. Although the portable gamemachine illustrated in FIG. 52A includes two display portions, thedisplay portion 7103 and the display portion 7104, the number of displayportions included in the portable game machine is not limited to two.

FIG. 52B illustrates a smart watch, which includes a housing 7302, adisplay portion 7304, operation buttons 7311 and 7312, a connectionterminal 7313, a band 7321, a clasp 7322, and the like. Thesemiconductor device of one embodiment of the present invention can beused for a memory, a CPU, or the like incorporated in the housing 7302.

FIG. 52C illustrates a portable information terminal, which includes adisplay portion 7502 incorporated in a housing 7501, operation buttons7503, an external connection port 7504, a speaker 7505, a microphone7506, and the like. The semiconductor device of one embodiment of thepresent invention can be used for a memory for mobile use, a CPU, or thelike incorporated in the housing 7501. Note that the display portion7502 is small- or medium-sized but can perform full high vision, 4 k, or8 k display because it has greatly high definition; therefore, asignificantly clear image can be obtained.

FIG. 52D illustrates a video camera, which includes a first housing7701, a second housing 7702, a display portion 7703, operation keys7704, a lens 7705, a joint 7706, and the like. The operation keys 7704and the lens 7705 are provided for the first housing 7701, and thedisplay portion 7703 is provided for the second housing 7702. The firsthousing 7701 and the second housing 7702 are connected to each otherwith the joint 7706, and the angle between the first housing 7701 andthe second housing 7702 can be changed with the joint 7706. Imagesdisplayed on the display portion 7703 may be switched in accordance withthe angle at the joint 7706 between the first housing 7701 and thesecond housing 7702. The imaging device of one embodiment of the presentinvention can be used in a portion corresponding to a focus of the lens7705. The semiconductor device of one embodiment of the presentinvention can be used for an integrated circuit, a CPU, or the likeincorporated in the first housing 7701.

FIG. 52E illustrates a digital signage, which includes a display portion7922 provided on a utility pole 7921. The semiconductor device of oneembodiment of the present invention can be used for a control circuit ofthe display portion 7922.

FIG. 53A illustrates a laptop personal computer, which includes ahousing 8121, a display portion 8122, a keyboard 8123, a pointing device8124, and the like. The semiconductor device of one embodiment of thepresent invention can be used for a CPU, a memory, or the likeincorporated in the housing 8121. Note that the display portion 8122 issmall- or medium-sized but can perform 8 k display because it hasgreatly high definition; therefore, a significantly clear image can beobtained.

FIG. 53B is an external view of an automobile 9700. FIG. 53C illustratesa driver's seat of the automobile 9700. The automobile 9700 includes acar body 9701, wheels 9702, a dashboard 9703, lights 9704, and the like.The semiconductor device of one embodiment of the present invention canbe used in a display portion and a control integrated circuit of theautomobile 9700. For example, the display device or semiconductor deviceof one embodiment of the present invention can be used in displayportions 9710 to 9715 illustrated in FIG. 53C.

The display portion 9710 and the display portion 9711 are displaydevices or input/output devices provided in an automobile windshield.The display device or input/output device of one embodiment of thepresent invention can be a see-through display device or input/outputdevice, through which the opposite side can be seen, by using alight-transmitting conductive material for its electrodes. Such asee-through display device or input/output device does not hinderdriver's vision during the driving of the automobile 9700. Therefore,the display device or input/output device of one embodiment of thepresent invention can be provided in the windshield of the automobile9700. Note that in the case where a transistor or the like for drivingthe display device or input/output device is provided in the displaydevice or input/output device, a transistor having light-transmittingproperties, such as an organic transistor using an organic semiconductormaterial or a transistor using an oxide semiconductor, is preferablyused.

The display portion 9712 is a display device provided on a pillarportion. For example, the display portion 9712 can compensate for theview hindered by the pillar portion by showing an image taken by animaging unit provided on the car body. The display portion 9713 is adisplay device provided on a dashboard portion. For example, the displayportion 9713 can compensate for the view hindered by the dashboardportion by showing an image taken by an imaging unit provided on the carbody. That is, showing an image taken by an imaging unit provided on theoutside of the car body leads to elimination of blind areas andenhancement of safety. In addition, showing an image so as to compensatefor the area which a driver cannot see makes it possible for the driverto confirm safety easily and comfortably.

FIG. 53D illustrates the inside of an automobile in which a bench seatis used as a driver seat and a front passenger seat. A display portion9721 is a display device or input/output device provided in a doorportion. For example, the display portion 9721 can compensate for theview hindered by the door portion by showing an image taken by animaging unit provided on the car body. A display portion 9722 is adisplay device provided in a steering wheel. A display portion 9723 is adisplay device provided in the middle of a seating face of the benchseat. Note that the display device can be used as a seat heater byproviding the display device on the seating face or backrest and byusing heat generated by the display device as a heat source.

The display portion 9714, the display portion 9715, and the displayportion 9722 can display a variety of kinds of information such asnavigation data, a speedometer, a tachometer, a mileage, a fuel meter, agearshift indicator, and air-condition setting. The content, layout, orthe like of the display on the display portions can be changed freely bya user as appropriate. The information listed above can also bedisplayed on the display portions 9710 to 9713, 9721, and 9723. Thedisplay portions 9710 to 9715 and 9721 to 9723 can also be used aslighting devices. The display portions 9710 to 9715 and 9721 to 9723 canalso be used as heating devices.

FIG. 54A illustrates an external view of a camera 8000. The camera 8000includes a housing 8001, a display portion 8002, an operation button8003, a shutter button 8004, a connection portion 8005, and the like. Alens 8006 can be put on the camera 8000.

The connection portion 8005 includes an electrode to connect a finder8100, which is described below, a stroboscope, or the like.

Although the lens 8006 of the camera 8000 here is detachable from thehousing 8001 for replacement, the lens 8006 may be included in thehousing 8001.

Images can be taken at the press of the shutter button 8004. Inaddition, images can be taken at the touch of the display portion 8002which serves as a touch panel.

The display device or semiconductor device of one embodiment of thepresent invention can be used in the display portion 8002.

FIG. 54B illustrates the camera 8000 with the finder 8100 connected.

The finder 8100 includes a housing 8101, a display portion 8102, abutton 8103, and the like.

The housing 8101 includes a connection portion for engagement with theconnection portion 8005 of the camera 8000 so that the finder 8100 canbe connected to the camera 8000. The connection portion includes anelectrode, and an image or the like received from the camera 8000through the electrode can be displayed on the display portion 8102.

The button 8103 has a function of a power button, and the displayportion 8102 can be turned on and off with the button 8103.

The semiconductor device of one embodiment of the present invention canbe used for an integrated circuit and an image sensor included in thehousing 8101.

Although the camera 8000 and the finder 8100 are separate and detachableelectronic devices in FIGS. 54A and 54B, the housing 8001 of the camera8000 may include a finder having the display device or input/outputdevice of one embodiment of the present invention.

FIG. 54C illustrates an external view of a head-mounted display 8200.

The head-mounted display 8200 includes a mounting portion 8201, a lens8202, a main body 8203, a display portion 8204, a cable 8205, and thelike. The mounting portion 8201 includes a battery 8206.

Power is supplied from the battery 8206 to the main body 8203 throughthe cable 8205. The main body 8203 includes a wireless receiver or thelike to receive video data, such as image data, and display it on thedisplay portion 8204. The movement of the eyeball and the eyelid of auser are captured by a camera in the main body 8203 and then coordinatesof the points the user looks at are calculated using the captured datato utilize the eye of the user as an input means.

The mounting portion 8201 may include a plurality of electrodes so as tobe in contact with the user. The main body 8203 may be configured tosense current flowing through the electrodes with the movement of theuser's eyeball to recognize the direction of his or her eyes. The mainbody 8203 may be configured to sense current flowing through theelectrodes to monitor the user's pulse. The mounting portion 8201 mayinclude sensors, such as a temperature sensor, a pressure sensor, or anacceleration sensor so that the user's biological information can bedisplayed on the display portion 8204. The main body 8203 may beconfigured to sense the movement of the user's head or the like to movean image displayed on the display portion 8204 in synchronization withthe movement of the user's head or the like.

The semiconductor device of one embodiment of the present invention canbe used for an integrated circuit included in the main body 8203.

This embodiment can be combined as appropriate with any of the otherembodiments in this specification.

Embodiment 11

In this embodiment, application examples of an RF tag using thesemiconductor device of one embodiment of the present invention will bedescribed with reference to FIGS. 55A to 55F.

<Application Examples of RF Tag>

The RF tag is widely used and can be provided for, for example, productssuch as bills, coins, securities, bearer bonds, documents (e.g.,driver's licenses or resident's cards, see FIG. 55A), vehicles (e.g.,bicycles, see FIG. 55B), packaging containers (e.g., wrapping paper orbottles, see FIG. 55C), recording media (e.g., DVD or video tapes, seeFIG. 55D), personal belongings (e.g., bags or glasses), foods, plants,animals, human bodies, clothing, household goods, medical supplies suchas medicine and chemicals, and electronic devices (e.g., liquid crystaldisplay devices, EL display devices, television sets, or cellularphones), or tags on products (see FIGS. 55E and 55F).

An RF tag 4000 of one embodiment of the present invention is fixed to aproduct by being attached to a surface thereof or embedded therein. Forexample, the RF tag 4000 is fixed to each product by being embedded inpaper of a book, or embedded in an organic resin of a package. Since theRF tag 4000 of one embodiment of the present invention can be reduced insize, thickness, and weight, it can be fixed to a product withoutspoiling the design of the product. Furthermore, bills, coins,securities, bearer bonds, documents, or the like can have anidentification function by being provided with the RF tag 4000 of oneembodiment of the present invention, and the identification function canbe utilized to prevent counterfeiting. Moreover, the efficiency of asystem such as an inspection system can be improved by providing the RFtag of one embodiment of the present invention for packaging containers,recording media, personal belongings, foods, clothing, household goods,electronic devices, or the like. Vehicles can also have higher securityagainst theft or the like by being provided with the RF tag of oneembodiment of the present invention.

As described above, by using the RF tag including the semiconductordevice of one embodiment of the present invention for each applicationdescribed in this embodiment, power for operation such as writing orreading of data can be reduced, which results in an increase in themaximum communication distance. Moreover, data can be held for anextremely long period even in the state where power is not supplied;thus, the RF tag can be preferably used for application in which data isnot frequently written or read.

This embodiment can be combined as appropriate with any of the otherembodiments in this specification.

This application is based on Japanese Patent Application serial no.2015-032252 filed with Japan Patent Office on Feb. 20, 2015, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A semiconductor device comprising: a firstinsulating layer; a first oxide layer over the first insulating layer; asemiconductor layer over the first oxide layer; a source electrode layerand a drain electrode layer over the semiconductor layer; a secondinsulating layer over the first insulating layer; a third insulatinglayer over the second insulating layer, the source electrode layer, andthe drain electrode layer; a second oxide layer over the semiconductorlayer; a gate insulating layer over the second oxide layer; a gateelectrode layer over the gate insulating layer; and a fourth insulatinglayer over the third insulating layer, the second oxide layer, the gateinsulating layer, and the gate electrode layer, wherein the secondinsulating layer includes a region in contact with a first side surfaceof the first oxide layer, a side surface of the semiconductor layer, afirst side surface of the source electrode layer, and a first sidesurface of the drain electrode layer, wherein a top surface of thesecond insulating layer is at the same level as a top surface of thesource electrode layer and a top surface of the drain electrode layer,and wherein the second oxide layer includes a region in contact with asecond side surface of the first oxide layer, a second side surface ofthe source electrode layer, a second side surface of the drain electrodelayer, a side surface of the second insulating layer, and a side surfaceof the third insulating layer.
 2. The semiconductor device according toclaim 1, wherein one of the first insulating layer, the secondinsulating layer, the third insulating layer, and the fourth insulatinglayer contains oxygen.
 3. The semiconductor device according to claim 1,wherein one of the first insulating layer, the second insulating layer,the third insulating layer, and the fourth insulating layer comprisessilicon oxide.
 4. The semiconductor device according claim 1 comprising:any one of a microphone, a speaker and a housing.
 5. The semiconductordevice according to claim 1, wherein one of the second insulating filmand the third insulating film comprises a low-k material.
 6. A methodfor manufacturing a semiconductor device comprising the steps of:forming a first insulating layer; forming a first oxide film over thefirst insulating layer; forming a semiconductor film over the firstoxide film; forming a first conductive film over the semiconductor film;forming a first mask over the first conductive film; partly etching thefirst conductive film with the first mask to form a first conductivelayer in an island shape; partly etching the first oxide film and thesemiconductor film with the first mask and the first conductive layerserving as a mask to form a first oxide layer and a semiconductor layerin island shapes; forming a second insulating film over the firstinsulating layer and the first conductive layer; performing a chemicalmechanical polishing process on the second insulating film until thefirst conductive layer is exposed to form a second insulating layer;forming a third insulating film over the first conductive layer and thesecond insulating layer; forming a second mask over the third insulatingfilm; partly etching the third insulating film with the second mask toform a source electrode layer, a drain electrode layer, and a thirdinsulating layer; forming a second oxide film over the third insulatinglayer and the semiconductor layer; forming a fourth insulating film overthe second oxide film; forming a second conductive film over the fourthinsulating film; and performing a chemical mechanical polishing processon the second conductive film, the fourth insulating film, and thesecond oxide film to form a second oxide layer, a gate insulating layer,and a gate electrode layer.
 7. The method for manufacturing asemiconductor device, according to claim 6, wherein first heat treatmentis performed after the semiconductor film is formed, wherein a fourthinsulating layer containing oxygen is formed over the third insulatinglayer, the second oxide layer, the gate insulating layer, and the gateelectrode layer, wherein a mixed layer of the third insulating layer andthe fourth insulating layer is formed when the fourth insulating layeris formed and, at the same time, oxygen is added to the mixed layer orthe first insulating layer, and wherein second heat treatment isperformed to diffuse the oxygen added to the mixed layer or the firstinsulating layer to the semiconductor layer.
 8. The method formanufacturing a semiconductor device, according to claim 7, wherein thethird insulating film is an insulating film containing oxygen, andwherein the fourth insulating layer is formed by a sputtering methodwith an oxygen gas.
 9. The method for manufacturing a semiconductordevice, according to claim 7, wherein the third insulating film is asilicon oxide film, and wherein the fourth insulating layer is formed bya sputtering method with an oxygen gas of 50 vol % or higher and analuminum oxide target.
 10. The method for manufacturing a semiconductordevice, according to claim 7, wherein the second heat treatment isperformed at a temperature higher than or equal to 300° C. and lowerthan or equal to 450° C.